percolate REST API should support source parameter

As stated in documentation, we should support `?source=` parameter in percolate REST operations.

This is how to reproduce it:

```sh
curl -XDELETE "http://localhost:9200/test"

curl -XPUT "http://localhost:9200/test/.percolator/1" -d'
{
"query" : {
"match" : {
"foo" : "bar"
}
}
}'

# This one works
curl -XPOST "http://localhost:9200/test/message/_percolate" -d '{
"doc" : {
"foo" : "bar is in foo"
}
}'

# This one gives: BroadcastShardOperationFailedException[[test][2] ]; nested: PercolateException[failed to percolate]; nested: ElasticsearchIllegalArgumentException[Nothing to percolate];
curl -XGET "http://localhost:9200/test/message/_percolate?source=%7B%22doc%22%3A%7B%22foo%22%3A%22bar%20is%20in%20foo%22%7D%7D"
```

Closes #4903.

src/main/java/org/elasticsearch/rest/action/percolate/RestPercolateAction.java

@@ -23,6 +23,8 @@
 import org.elasticsearch.action.percolate.PercolateRequest;
 import org.elasticsearch.action.percolate.PercolateResponse;
 import org.elasticsearch.client.Client;
+import org.elasticsearch.common.bytes.BytesArray;
+import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.inject.Inject;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.xcontent.XContentBuilder;
@@ -52,7 +54,18 @@ public RestPercolateAction(Settings settings, Client client, RestController cont
     public void handleRequest(final RestRequest request, final RestChannel channel) {
         PercolateRequest percolateRequest = new PercolateRequest(request.param("index"), request.param("type"));
         percolateRequest.listenerThreaded(false);
-        percolateRequest.source(request.content(), request.contentUnsafe());
+
+        BytesReference content = null;
+        if (request.hasContent()) {
+            content = request.content();
+        } else {
+            String source = request.param("source");
+            if (source != null) {
+                content = new BytesArray(source);
+            }
+        }
+
+        percolateRequest.source(content, request.contentUnsafe());
 
         // we just send a response, no need to fork
         percolateRequest.listenerThreaded(false);