Updated Address.String so it restores quoted local parts, which wasn't
done before.
When parsing `<" "@example.com>`, the formatted string returned
`< @example>`, which doens't match RFC 5322, since a space is not atext.
Another example is `<"bob@valid"@example.com>` which returned
`<bob@valid@example.com>`, which is completely invalid.
I also added support for quotes and backslashes in a quoted local part.

Besides formatting a parsed Address, the ParseAddress function also
needed more testing and finetuning for special cases.
Things like `<.john.doe@example.com>` and `<john..doe@example.com>`
e.a. were accepted, but are invalid.
I fixed those details and add tests for some other special cases.

Fixes #10768

Change-Id: Ib0caf8ad603eb21e32fcb957a5f1a0fe5d1c6e6e
Reviewed-on: https://go-review.googlesource.com/8724Reviewed-by: Russ Cox <rsc@golang.org>