support Feign.builder() w/ SAX decoder

simplified based on feedback from @davidmc24 yesterday.  Notably supports `Feign.builder()` now.

Ex.

```java
api = Feign.builder()
           .decoder(SAXDecoder.builder()
                              .registerContentHandler(UserIdHandler.class)
                              .build())
           .target(Api.class, "https://apihost");
```