Merge remote-tracking branch 'ac/header-params' into jw/rewrite-all-the-things

Conflicts:
	retrofit/src/main/java/retrofit/http/RequestBuilder.java
	retrofit/src/main/java/retrofit/http/RequestHeaders.java
	retrofit/src/main/java/retrofit/http/RestAdapter.java
	retrofit/src/main/java/retrofit/http/RestMethodInfo.java
	retrofit/src/main/java/retrofit/http/client/ApacheClient.java
	retrofit/src/main/java/retrofit/http/client/Header.java
	retrofit/src/main/java/retrofit/http/client/Request.java
	retrofit/src/main/java/retrofit/http/client/Response.java
	retrofit/src/main/java/retrofit/http/client/UrlConnectionClient.java
	retrofit/src/test/java/retrofit/http/RequestBuilderTest.java
	retrofit/src/test/java/retrofit/http/client/ApacheClientTest.java
	retrofit/src/test/java/retrofit/http/client/UrlConnectionClientTest.java