Mention that headers can be passed as an array. Fixes https://github.com/andris9/Nodemailer/issues/334.
Merge pull request #335 from bradvogel/master

Mention that headers can be passed as an array. Fixes https://github.com...
Fix incorrect docs.
Merge pull request #337 from bradvogel/master

Fix incorrect docs.
Added a note about using Gmail
Merge branch 'master' of github.com:andris9/Nodemailer
Updated sendgrid link –added referral code
Adding the acceptance of a sender header from options.
Fixing typo
Fixing whitespace
Ordering things more logically.
Merge pull request #351 from TylerDixon/master

Added sender header field