Implement sliding window sync.

This change allows null QueryParams to be skipped.
diff --git a/core/java/com/google/android/gdata/client/QueryParamsImpl.java b/core/java/com/google/android/gdata/client/QueryParamsImpl.java
index e27b36f..fbe0d22 100644
--- a/core/java/com/google/android/gdata/client/QueryParamsImpl.java
+++ b/core/java/com/google/android/gdata/client/QueryParamsImpl.java
@@ -60,6 +60,8 @@
             sb.append('?');
         }
         for (String param : params) {
+            String value = mParams.get(param);
+            if (value == null) continue;
             if (first) {
                 first = false;
             } else {
@@ -67,7 +69,7 @@
             }
             sb.append(param);
             sb.append('=');
-            String value = mParams.get(param);
+
             String encodedValue = null;
 
             try {