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.
Remove sorting headers from library code. The application must sort
them if necessary. nghttpx and nghttpd do the sorting of the headers
in stable way if names are equal.