By default, as RFC 7540 calls for, pushed stream depends on its
associated (parent) stream.  There are some situations that this is
sub-optimal.  For example, if associated stream is HTML, and server is
configured to push css and javascript files which are in critical
rendering path.  Then the default priority scheme is sub-optimal,
since browser typically blocks rendering while waiting for critical
resources.  In this case, it is better to at least give pushed stream
the same priority of associated stream, and interleave these streams.

This change gives pushed stream the same priority of associated stream
if pushed stream has content-type "application/javascript" or
"text/css".  The pushed stream now depends on the stream which
associated stream depends on.  We use the same weight of associated
stream.