The existing code has partial support for -07 (just the hours of a time
zone offset).  Complete the support, add support for Z07, and add a few
tests.

Fixes #13426.

Change-Id: Ic6377bbf3e65b4bb761b9779f7e80c07ce4f57e8
Reviewed-on: https://go-review.googlesource.com/17260Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>