Quite disappointingly OkHttp 3.8's new requirement for a non-null message
ends up breaking Retrofit. Good excuse to get this release out at the
same time.

This adds support for automatic BOM handling when character-based mechanisms are used on ResponseBody.

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

Use a more idiomatic creation mechanism which eliminates a lot of responsibility for our implementation.

TestSubscriber is a poor implementation for correctness testing. This new subscriber ensures all events are tested by being a JUnit rule. It also consumes events as they're validated so that assertion becomes more like a script than a ledger.

This also switches tests to use unsafeSubscribe to ensure that any contract violations are not being suppressed. The latest version of RxJava is required for accessing this method on Completable.

Update to latest okhttp release.

This allows settings like support for leniency to be honored.