fix: client proxy incorrectly resets "Accept" header defined by @Produces annotation

https://java.net/jira/browse/JERSEY-2696
I know directly from the JVM team that when they analyse any more complex code, they tend to see a large number of empty array initializations that reduce performance. Thus my earlier comment.

Overall, it is better to have a single statically allocated empty reference, that consumes 4-8 bytes (depending on whether [OOPs](http://docs.oracle.com/javase/7/docs/technotes/guides/vm/performance-enhancements-7.html#compressedOop) are compressed or not), than to potentially create a new empty array instance with every single request.