RESTEASY-1106: ResponseBuilderImpl.links() removes previously added Link-Headers

This fixes the links()-implementation to reset the Link-headers only if
null is provided as argument - as specified in the Interface.

Simple test is included.
Merge pull request #569 from ja-fra/RESTEASY-1106

RESTEASY-1106: ResponseBuilderImpl.links() removes previously added Link-Headers
RESTEASY-1107
Merge pull request #570 from patriot1burke/master

RESTEASY-1107
setRequestUri fixes
Merge pull request #571 from patriot1burke/master

setRequestUri fixes