Adding support for HTTP/2 binary headers

Motivation:

The HTTP/2 spec does not restrict headers to being String. The current
implementation of the HTTP/2 codec uses Strings as header keys and
values. We should change this so that header keys and values allow
binary values.

Modifications:

Making Http2Headers based on AsciiString, which is a wrapper around a
byte[].
Various changes throughout the HTTP/2 codec to use the new interface.

Result:

HTTP/2 codec no longer requires string headers.