Ignore the first and last "parts" in multipart messages

[GREsau]

In my experience, MailHog's MIME tab displays every email as having an empty initial part and a 2-byte final part (consisting of 2 dashes). This is because MailHog is incorrectly treating content before the initial boundary and after the final boundary as real parts. This content should be ignored, as specified by RFC 2046:

There appears to be room for additional information prior to the
first boundary delimiter line and following the final boundary
delimiter line. These areas should generally be left blank, and
implementations must ignore anything that appears before the first
boundary delimiter line or after the last one.

This PR will make MailHog ignore everything before the first multipart boundary and after the last one.

[g-regor]

In my case I don't see any empty parts. Might not be wise blindly removing parts.