Fix a race closing MockSpdyPeer.

It would have crashed if close() was called before readAndWriteFrames()
started.
It causes the getResponseHeadersTimesOut fail when the closeCancelsPings
has been crashed by the MockSpdyPeer's defect.
This patch was referred by below commit:
https://github.com/square/okhttp/commit/d2d8288bb0aded0a0a6fae9e38818744378fbe45



Change-Id: Icffdf83221d6688a588d026eb1b986cd26d81f88
Signed-off-by: Takeshi Katae <takeshi.katae.xm@kyocera.jp>