Update Robolectric

Update OkHttp to 3.12

Enable built-in CompletableFuture adapter on Android API 24+

Add some missing Nullable annotations

Add tests for form and multipart header values and overrides

Add some missing Nullable annotations

Enable built-in Optional converter on Android API 24+

Use a dummy value to further validation optional delegation

Prior to this other things might cause a null value. This ensures the value can only come from only place.

Forbid path traversal ('.' and '..') as @Path parameters.

They're likely to have the unintended effect. For example, passing ".."
to @DELETE /account/book/{isbn}/ yields @DELETE /account/.

Update Proguard rules for R8 fullmode

Taken from here https://github.com/JakeWharton/SdkSearch/commit/498ab309cf83e47e2546379dc1c59eac27711bec

Replace @ParametersAreNonnullByDefault with @EverythingIsNonNull

Fix Optional converter factories not delegating properly.

Move Optional support into Retrofit core.

Test all Rx types can be subscribed to twice

Fix Javadoc formatting.

Move CompletableFuture support into Retrofit core.

Expose static factory for Invocation

Also rework the internals of how an Invocation gets instantiated and set as a tag on the Request. This will make a future change easier to land.

New 'Invocation' class for tracking metrics on Retrofit calls