Updated Query Expressions to support empty and undefined values

[kdavisk6]

Fixes #872

Previously, all unresolved query template expressions resolved
to empty strings, which then indcate that the entire query parameter
should be removed. This violates RFC 6570 in that only undefined
values should be removed. This change updates Query Template to
check the provided variables map for an entry expression. If
no value is provided, the entry is explicitly marked UNDEF and
removed.

This brings us in line with the specification. The following is
now how parameters are resolved:

Empty String

public void test() {
   Map<String, Object> parameters = new LinkedHashMap<>();
   parameters.put("param", "");
   this.demoClient.test(parameters);
}

Result

http://localhost:8080/test?param=

Missing

public void test() {
   Map<String, Object> parameters = new LinkedHashMap<>();
   this.demoClient.test(parameters);
}

Result

http://localhost:8080/test

Undefined

public void test() {
   Map<String, Object> parameters = new LinkedHashMap<>();
   parameters.put("param", null);
   this.demoClient.test(parameters);
}

Result

http://localhost:8080/test

• Adding additional test case for explicit null parameter value

• Additional Test case for the explict null case. Updates to the
  documentation.