flate's reader greedily reads from the shared io.Reader in Framer. This leads to a data race on Framer.r. Fix this by providing a corkedReader to zlib.NewReaderDict(). We uncork the reader and allow it to read the number of bytes in the compressed payload.

Fixes #1884.

R=bradfitz, rsc, go.peter.90
CC=golang-dev
https://golang.org/cl/4530089