Force a full read of header values

For some reason, on stream id 151 when talking to Facebook, `s.out.Read` was not reading a full header value, causing a parsing error.

I suppose you should be doing `io.ReadFull` in more places, but this fixed my issue, so...

It would be nice to find out why it was always happening on the same header of stream 151. Could it be some buffer sizing thing?