Fix erroneous (but not fatal) calculation in HuffmanAppend
Add an explanatory comment to Http2Compressor::EncodeHeaderBlock so I don't keep getting confused
Add logging to Http2Compressor::HuffmanAppend
Fix off-by-one in Http2Stream::ParseHttpRequestHeaders

This caused our window update to trample over the last byte of the last
header value in our initial HEADERS frame. Also make the comments around
this area more explanatory, so hopefully something like this doesn't
happen again.