Delete entity body for some GET requests

If a GET request includes an entity body that is an empty object, delete it.

Some servers will reject GET requests with an entity body, as it is non-standard. Also, Request automatically adds a content-length header to requests with an entity body, which can cause unpredictable effects.

This change won't affect developers who intentionally include an entity-body, as it only deletes empty objects.

It is possible to implement this using a custom data-adapter, but I think it makes sense for this to be the default behavior, as many servers don't expect an entity body in a GET request.

Fixes #87.