To work around a bug in SwitfMailer the character set of a message part
should always be set. When no character set is explicitly defined use
the renderCharset instead.

Change-Id: I712128156ee86e1d2372fef826703fb35a3b1122
Fixes: #28684
Releases: 4.7, 4.6, 4.5
Reviewed-on: http://review.typo3.org/6744
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Philipp Gampe
Reviewed-by: Jigal van Hemert
Tested-by: Jigal van Hemert