Implement sliding window sync.

Add support for null QueryParams.
diff --git a/src/com/google/wireless/gdata/client/HttpQueryParams.java b/src/com/google/wireless/gdata/client/HttpQueryParams.java
index 4878002..e391e27 100644
--- a/src/com/google/wireless/gdata/client/HttpQueryParams.java
+++ b/src/com/google/wireless/gdata/client/HttpQueryParams.java
@@ -43,8 +43,10 @@
         url.append('&');
       }
       String name = (String) names.elementAt(i);
+      String value = getParamValue(name);
+      if (value == null) continue;
       url.append(client.encodeUri(name)).append('=');
-      url.append(client.encodeUri(getParamValue(name)));
+      url.append(client.encodeUri(value));
     }
     return url.toString();
   }