Implement toString for OutputType.

Returns a String that textually represents the object. I desire
this for viewing in a debugger and for user code that logs
OutputType.

Signed-off-by: Seva Lotoshnikov <vlotoshnikov@gmail.com>

java/client/src/org/openqa/selenium/OutputType.java

@@ -43,6 +43,10 @@ public String convertFromBase64Png(String base64Png) {
     public String convertFromPngBytes(byte[] png) {
       return new Base64Encoder().encode(png);
     }
+
+    public String toString() {
+      return "OutputType.BASE64";
+    }
   };
 
   /**
@@ -56,6 +60,10 @@ public byte[] convertFromBase64Png(String base64Png) {
     public byte[] convertFromPngBytes(byte[] png) {
       return png;
     }
+
+    public String toString() {
+      return "OutputType.BYTES";
+    }
   };
 
   /**
@@ -94,6 +102,10 @@ private File save(byte[] data) {
         }
       }
     }
+
+    public String toString() {
+      return "OutputType.FILE";
+    }
   };
 
   /**