Merge headers on send

[Swaagie]

Headers options are currently not merged. Specifying

headers: {    
  'Reply-To': body.email
}

will send a plain text e-mail since the content-type is overruled. Doing a deep merge should fix this issue as it still allows overruling default headers.

[Swaagie]

Ignore my ramblings, it failed due to not escaping the generated HTML from markdown