Cleanup and test exposed utility behavior.

Correct rendering of '@' in Javadoc.

Pass the wrapped call to callbacks.

This ensures behavior like `clone()` retain the correct beahvior.

Eliminate some allocation/state overhead by implementing subscription directly.

Adapter Javadoc details.

Call's internal volatile boolean for tracking canceled was already mirroring this state so it can be used directly.

Support supplying a protobuf extension registry for deserialization.

Support iterable and array-based OkHttp Part parts.

Also forbid OkHttp Part parts in iterables and arrays when a part name was specified.

Honor backpressure in Observable creation.

Document maps don't allow null anywhere.

Retrofit web crawler example.

Support HttpUrl with @url.

Add support for OkHttp's MultipartBody.Part.

To do this we need to make `@Part`'s value (i.e., name) optional, since `MultipartBody.Part` brings its name.

`MultipartBody.Part` is a replacement for Retrofit 1's `TypedOutput`/`TypedFile` for doing uploads of files with names.

Remove the need for generated accessor methods.

Null check Call.enqueue argument.

Sample for a dynamic base URL.