HttpServletRequest.getHeader returns null for empty header in jetty 9

[btd]

If pass in header
User-Agent:
HttpServletRequest.getHeader("User-Agent") will return null in jetty 9.

There is a groups discussion about it:
https://groups.google.com/forum/#!topic/liftweb/9712-b4lOB8

And reply about this issue jetty developers:
http://dev.eclipse.org/mhonarc/lists/jetty-users/msg03339.html

So it is defined behaviour and will not be changed by developers.