Change spdy and http/2 headers to always lowercase

Change spdy and http/2 headers to always lowercase header names.  This avoids allocation and shortens runtime of header name comparison.  It does so by down-casing header names as bytes are read from the server.  With this in mind, the case-insensitive comparison only needs to concern itself with mixed case of the compared header.

I profiled case-insensitive comparison on both sides vs comparing lowercase to mixed case and found the latter between 3.4 and 9.2% faster runtime.