Fix set body length in DKIM
merged branch Scimon/master (PR #281)

This PR was merged into the master branch.

Commits
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ebc6bd8 Fix set body length in DKIM

Discussion
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Small fix

Hi, I've been using Swiftmailer recently for DKIM signing and noticed the setBodyLength function in the DKIM signer was incorrectly referencing $this->len not the $len argument.

Cheers.

Simon