The existing implementation doesn't send proper rfc1341 compatible mails
according to https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html as
the closing `--boundary_id--` is missing from our mails. (see 7.2.1 from
the standard).
Mailservers which are not very flexible, deny the mails.
Example mail log: (see the
missing boundary delimiter at the end).
```
Subject: Activate your email
From: "Storj DCS - EU1" <noreply@eu1.storj.io>
To: "..." <...>
...
--26d7220f6f1c9f6fb47b535319eb15dce513bb6e1d941a0efddf25e96712
Content-Type: text/html; charset=UTF-8
....
</body>
</html>
.
smtp: DATA error {"msg_id":"6d82c9e3","reason":"unexpected EOF"}
smtp: 554 5.0.0 Internal server error (msg ID = 6d82c9e3)
```
This patch moves all the Multipart writing to a function to make sure
that the `wr.Close()` (which writes out the last part) is executed
BEFORE `body.Bytes()` (defer added it AFTER `body.Bytes()` was calculated)
Change-Id: I8f18fc81b1857b646470eab32e73d6cbdc50d2ad
