This happens when deflater local header table is smaller than
inflater header table size. We use the same notation
"**DEALLOCATED**" for such name/values with hdtest programs.

We thought that this kind of rewrite can be achieved by the configuration
of the backend severs, but in some configuration, however, it may get
complicated. So we decided to implement at least location rewrite in
nghttpx.

This commit also contains a fix to the bug which prevents the http2
backend request from concatenating header fields with the same value.