net/http: TimeoutHandler doesn't set a timeout on Request.Context

[jmhodges]

On Go version 1.8.3, net/http.TimeoutHandler doesn't apply context.WithTimeout to the passed in http.Request's context. It's desirable now that context is everywhere in the stdlib. It seems like it might also simplify the timeoutHandler.ServeHTTP implementation.

This also seems to be the case in 1.9beta1.

[odeke-em]

/cc @bradfitz

[gopherbot]

CL https://golang.org/cl/46412 mentions this issue.