Add support for HEAD requests of resources

Allow Resource.head* to make HEAD requests. When header data comes back,
X-Trans-Id headers are converted into 'id' data to match the rest of
the response types, and to match Resource.id attribute access.

Change-Id: I6d898f0734bbdbcb02b79d3387ee8248e401a36e
Merge "Add support for HEAD requests of resources"