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

https://java.net/jira/browse/JERSEY-2696
Accepted. However, food for thought:

Personally, I am also careful and reluctant to create constants like this. They create  (accumulated) initialization overhead and noncollectable garbage on heap.

My simple object allocation was cheap, stack-scoped, easily GC-able. It was intentional.

That's my point of view, JVM experts instead of architects might have a say here.