JERSEY-2562: Invocation.Builder#header(name, null) violates specification

 - fix + test

Change-Id: Idf92404ae0e244aeecac63391856e1007cb71e84
Signed-off-by: Michal Gajdos <michal.gajdos@oracle.com>
J-193: GitHub Review: 2495 & 2059

 - fixed copyrights

Change-Id: I52ce8f49fb15d5fa035236cf01a2a9d4466476cb
Signed-off-by: Michal Gajdos <michal.gajdos@oracle.com>
Merge "JERSEY-2562: Invocation.Builder#header(name, null) violates specification"