This function is intended for use when the content is a static const variable.
It allows gzipped static data, but doesn't provide the functionality to ungzip
it if the client doesn't support gzip. This is because it would add a
dependency on zlib and basically all clients support gzip.
Change extraHeader on all Send functions to include the final newline,
this makes it easier to build up extra headers incrementally.
Expose sig::Connection for messageComplete and headerConn to allow them to
be disconnected by users of the class. This is commonly needed for things
like WebSocket upgrades.
