Fix UTF-8 in email

Summary:
Email was not being sent with the right headers/encoding for UTF-8.

Test Plan:
Sent UTF-8 mail using SES, default and SendGrid adapters. SendGrid already
worked; SES and default share the same code so this fixes both.

Reviewed By: slawekbiel
Reviewers: slawekbiel, aran, jungejason, tuomaspelkonen
CC: aran, slawekbiel
Differential Revision: 401