Update to Gson 2.8.2.

Add initial capacity for converter factory list.

* Add Retrofit.Builder.converterFactories().

* Don't shift N converter factories.

* Assert clone build exposes no interceptors.

* Remove implementation detail test for when converters are added.

* Ensure void response body handled by built-in converters.

* Remove builtInConvertersRemainFirstInClone.

* Test built in converters remain first in clone.

Remove volatile modifier from synchronized field.

Update to Gson 2.8.0.

Update to OkHttp 3.8.1.

Update to Moshi 1.5.0.

Add simple usage docs to each adapter.

Remove code duplication

Clarify that errors start at 0% probability.

Add some missing nullable annotations.

Add ProGuard rules

Fix typos in changelog

Please read what you writing before commiting... Thanks

Upgrade to OkHttp 3.8.

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.

Explicitly handle wrapped, re-thrown exceptions from onComplete, onError.

Allowing skipping of params if the converters converted to a null

Add converters which facilitate the use of Optional.

Use custom exception type when triggering mock failure.

Since we omit the stack trace, this more clearly indicates the source being from Retrofit's mock behavior.

Both Java 8 and Guava are supported, just like in the adapters.

This is especially useful in the use of RxJava 2 as it does not allow null values in the stream. This means a return type declaration of Single<BodyType> might result in an NPE should a converter deserialize the response body into null. By adding one of these delegating converters before your normal converter, it allows you to use Single<Optional<BodyType>> to support this case.