Whitespace characters are allowed in quoted-string according to RFC 5322 without
being "Q"-encoding. Address.String() already always formats the name portion in
quoted string, so whitespace characters should be allowed in there.

Fixes #6641.

LGTM=dave, dsymonds
R=golang-codereviews, gobot, dsymonds, dave
CC=golang-codereviews
https://golang.org/cl/55770043