net/http: easier way to check whether an error is due to timeout

<pre>Currently the only way to check whether a Get/Post/Do failed due to the respond headers
taking too long is to check the error string for the proper contents.

It would help if http.responseAndError satisfied the net.Error interface so one could
check for err.Timeout() if you'd like to treat timeouts differently.

go version: 1.1.1</pre>
