rfc2047 says:
  White space between adjacent 'encoded-word's is not displayed.

Although, mime package already have that feature,
we cannot simply reuse that code,
because there is a subtle difference in quoted-string handling.

Fixes #19363

Change-Id: I754201aa3c6b701074ad78fe46818af5b96cbd00
Reviewed-on: https://go-review.googlesource.com/37811Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>