Provide RequestTemplate.getParams() to access template variables (@Param) within an interceptor #601

public class MyServce {
   private final MyClient client;

   public void doSmth() {
      // Set the codec to some value. e.g. MP3. The key thing is that only the class "MyService"
      // knows which codec to choose.
      client.getSomething("MP3");
   }
}

public interface MyClient {
    // define the template variable "codec".
    // codec is not used in RequestLine, Body or Header
    @RequestLine("GET /")
    Object getSomething(@Param("codec") String codec);
}

public class CodecInterceptor implements RequestInterceptor {
    @Override
    public void apply(RequestTemplate requestTemplate) {
        // use params.get("codec") to get the codec and generate the headers
        Map<String, Object> params = requestTemplate.getParams();
        // ... 
    }
}