Fixing @return tag for getFrom method

The @return tag for the Swift_Mime_SimpleMessage::getFrom method should be mixed instead of string because both null (e.g., when the 'From' header is, for some reason, not found) or array (e.g., when multiple-from's are specified, other cases as well) may be returned.
minor #506 Fixing @return tag for getFrom method (terrycorley)

This PR was merged into the 5.2-dev branch.

Discussion
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Fixing @return tag for getFrom method

The @return tag for the Swift_Mime_SimpleMessage::getFrom method should be mixed instead of string because both null (e.g., when the 'From' header is, for some reason, not found) or array (e.g., when multiple-from's are specified, other cases as well) may be returned.

Commits
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c62ea93 Fixing @return tag for getFrom method