Also fix some Javadoc errors and ensure Javadoc (and source) jar generation is tested on CI.

From https://github.com/JakeWharton/retrofit2-rxjava2-adapter

This allows the mock implementation to only concern itself with the response type and the BehaviorDelegate will apply any delays from NetworkBehavior and also run through the Retrofit's CallAdapter.Factory list to turn the delayed Call into the appropriate type that matches the return type of the method.