Recognizes reason phrase is nullable and not set when using http/2

See httpwg/http2-spec#202
Fixes OpenFeign#382

Fixes dispatch for toString, equals, hashCode in hystrix

This copies missing logic from `FeignInvocationHandler` to
`HystrixInvocationHandler`, preventing NPEs when calling methods defined
on `java.lang.Object`.

Do not error on interfaces with default methods.

Pass calls to default methods on proxy through to implementation on interface.

Documents intent of client tests

Fixed version log

JAXRS: Fix NPE when @path is first on a method

Apache HTTP client hates to have the Content-Length set

It always sets the header itself and if we set it as well it barfs
so it is better just to avoid setting it at all.

support PUT with empty body parameter

follow-up commit to OpenFeign#271

Allows HystrixFeign.Builder to be used incrementally

Uses covariant overrides so that those collecting Feign configuration
via `Feign.Builder` can cast into `HystrixFeign.builder` to target an
api.

Closes OpenFeign#313

Fixes CHANGELOG