Fixes NPE when apache client rebuffers content

When log level is full, the response body is rebuffered. The Apache
client had a bug where it allowed `toInputStream` to return null. This
fixes that bug and backfills tests for the other two clients.

Fixes OpenFeign#255

Bumps changelog

Bumps dependency versions for integrations

* OkHttp/MockWebServer 2.4.0
* Gson 2.3.1
* Jackson 2.6.0
* Ribbon 2.1.0
* SLF4J 1.7.12

Avoids InputStream.available when determining if a stream is empty

InputStream.available isn't a reliable api. This uses an alternative
approach, which is to read the first byte to see if it is present. This
allows us to continue to avoid "No content to map due to end-of-input"
errors, but in a more supportable way.

Fixes OpenFeign#250

Adds base api support via single-inheritance interfaces

Before this change, apis that follow patterns across a service could
only be modeled by copy/paste/find/replace. Especially with a large
count, this is monotonous and error prone.

This change introduces support for base apis via single-inheritance
interfaces. Users ensure their target interface bind any type variables
and as a result have little effort to create boilerplate apis.

Ex.

```java
@headers("Accept: application/json")
interface BaseApi<V> {

  @RequestLine("GET /api/{key}")
  V get(@param("key") String);

  @RequestLine("GET /api")
  List<V> list();

  @headers("Content-Type: application/json")
  @RequestLine("PUT /api/{key}")
  void put(@param("key") String, V value);
}

interface FooApi extends BaseApi<Foo> { }

interface BarApi extends BaseApi<Bar> { }
```

closes OpenFeign#133

bump changelog

bump changelog

Fix regex support when there are multiple @path regexes

Fix an issue with trailing slashes in @path annotations on classes

Adds deflate encoding support