GCM: set Adapter Parameters does nothing

[greiter]

Hi, the method setAdapterParameters in the Gcm Adapter class does actually nothing, because the httpClient variable is not used in the getOpenedClient Function.

It would be nice if you could fix this, so that getOpenedClient Function takes the httpclient variable if set or otherwise create a new httpclient.

Many thanks.

[seyfer]

@greiter, please review latest changes in our master and make a PR if still needed.