Added headers manually to override error

[damo-da]

I was getting the following issue:
RuntimeError: Mailgun API error: 400 b'{\n "message": "\'from\' parameter is missing"\n}',

Providing custom header for Content-Type was the solution.

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[damo-da]

I signed it!

[googlebot]

CLAs look good, thanks!

[theacodes]

@underscoredam this looks good, but the lines are now too long. Can you break them up so they're under 80 characters?

[damo-da]

Thanks @jonparrott !

[theacodes]

@underscoredam no, thank you! 👍