Changed the behavior of rdf_serialize to accept a vector of prefix …

…to namespace bindings. (#22)

* Update rdf_serialize.R

 Changed the `rdf_serialize` function to accept multiple prefix to namespace mappings via the `namespace` parameter.
    The `namespace` parameter needs now to be a named character. `names(namespace)` contains the prefixes, whereas
    `namespace` itself contains the namespaces. Example:

```
    infile <- system.file("extdata", "dc.rdf", package="redland")
    out <- tempfile("file", fileext = ".rdf")
    some_rdf <- rdf_parse(infile)
    rdf_add(some_rdf, subject = "http://www.dajobe.org/dave-beckett", predicate = "http://www.w3.org/1999/02/22-rdf-syntax-ns#type", object = "http://xmlns.com/foaf/0.1/Person")
    rdf_serialize(some_rdf, out, format = "turtle", namespace = c(dc = "http://purl.org/dc/elements/1.1/", foaf = "http://xmlns.com/foaf/0.1/") )
    readLines(out)
```

* changes to documentation regarding  fix

* backward compatibility change

* fixed documentation as well

* added check for length(prefix) == length(namespace)

* added myself as contributor :)

DESCRIPTION

@@ -10,7 +10,11 @@ Authors@R: c(person("Carl", "Boettiger",
                   comment=c(ORCID = "0000-0002-0381-3766")),
              person("Anna", "Krystalli", 
                   role = "rev",
-                  comment=c(ORCID = "0000-0002-2378-4915")))                  
+                  comment=c(ORCID = "0000-0002-2378-4915")),
+             person("Viktor", "Senderov",
+                  email = "vsenderov@gmail.com",
+                  role = "ctb",
+                  comment = c(ORCID = "0000-0003-3340-5963")))
 Description: The Resource Description Framework, or 'RDF' is a widely used
              data representation model that forms the cornerstone of the 
              Semantic Web. 'RDF' represents data as a graph rather than 

R/rdf_serialize.R

@@ -3,8 +3,8 @@
 #' @inheritParams rdf_parse
 #' @inheritParams rdf_query
 #' @param doc file path to write out to. If null, will write to character.
-#' @param namespace string giving the namespace to set
-#' @param prefix string giving the prefix associated with the namespace
+#' @param namespace a named character containing the prefix to namespace bindings. \code{names(namespace)} are the prefixes, whereas \code{namespace} are the namespaces
+#' @param prefix (optional) for backward compatibility. See \code{namespace}. It contains the matching prefixes to the namespaces in \code{namespace} and is set automatically if you provide \code{namespace} as a named character vector.
 #' @param ... additional arguments to \code{redland::serializeToFile}
 #' @return rdf_serialize returns the output file path `doc` invisibly.
 #'   This makes it easier to use rdf_serialize in pipe chains with
@@ -19,15 +19,22 @@
 #' infile <- system.file("extdata", "dc.rdf", package="redland")
 #' out <- tempfile("file", fileext = ".rdf")
 #'
-#' rdf <- rdf_parse(infile)
-#' rdf_serialize(rdf, out)
+#' some_rdf <- rdf_parse(infile)
+#' rdf_add(some_rdf,
+#'     subject = "http://www.dajobe.org/dave-beckett",
+#'     predicate = "http://www.w3.org/1999/02/22-rdf-syntax-ns#type",
+#'     object = "http://xmlns.com/foaf/0.1/Person")
+#' rdf_serialize(some_rdf, out)
 #'
 #' ## With a namespace
-#' rdf_serialize(rdf,
+#' rdf_serialize(some_rdf,
 #'           out,
-#'           namespace = "http://purl.org/dc/elements/1.1/",
-#'           prefix = "dc")
+#'           format = "turtle",
+#'           namespace = c(dc = "http://purl.org/dc/elements/1.1/",
+#'           foaf = "http://xmlns.com/foaf/0.1/")
+#'           )
 #'
+#' readLines(out)
 rdf_serialize <- function(rdf,
                           doc = NULL,
                           format = c("guess",
@@ -37,7 +44,7 @@ rdf_serialize <- function(rdf,
                                      "turtle",
                                      "jsonld"),
                           namespace = NULL,
-                          prefix = NULL,
+                          prefix = names(namespace),
                           base = getOption("rdf_base_uri", "localhost://"),
                           ...){
 
@@ -59,11 +66,14 @@ rdf_serialize <- function(rdf,
     new("Serializer", rdf$world,
         name = format, mimeType = mimetype)
 
-  if(!is.null(namespace)){
-    redland::setNameSpace(serializer,
-                          rdf$world,
-                          namespace = namespace,
-                          prefix = prefix)
+  if(!is.null(namespace) && is.character(namespace) && length(namespace) >= 1 && length(namespace) == length(prefix)){
+    ix = 1:length(namespace)
+    for (i in ix) {
+      redland::setNameSpace(serializer,
+                            rdf$world,
+                            namespace = namespace[i],
+                            prefix = prefix[i]) 
+    }
   }
 
   if(is.null(doc)){