net/http: use TCP Keep-Alives on DefaultTransport's connections
net/http: use TCP Keep-Alives on DefaultTransport's connections

Update Issue 3362

Also set a 30 second timeout, instead of relying on the
operating system's timeout, which if often but not always 3
minutes.

LGTM=crawshaw
R=rsc, crawshaw
CC=golang-codereviews
https://codereview.appspot.com/68330046

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