messagePart.charset is not defined for Swift_Attachments

Fix for this error:

`Method "charset" for object "Swift_Attachment" does not exist in SwiftmailerBundle:Collector:swiftmailer.html.twig at line 61`
merged branch Burgov/patch-1 (PR #37)

This PR was merged into the master branch.

Discussion
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messagePart.charset is not defined for Swift_Attachments

Fix for this error:

`Method "charset" for object "Swift_Attachment" does not exist in SwiftmailerBundle:Collector:swiftmailer.html.twig at line 61`

Commits
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42d3de8 messagePart.charset is not defined for Swift_Attachments