Resolved JERSEY-1801.

- Fixed setting multiple header values in a servlet container (ResponseWriter)
- Added servlet-based test coverage
- Added test to verify the fix does not break MVC support (e.g. multiple response Content-Type headers etc.)

Change-Id: I449390155d8722ef80d8c17d3f637aa871daacab
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>