Outlook boundaries

[rinkp]

Fixes #6

[djmattyg007]

Could you please add a test case that would fail without this change?

[rinkp]

As you can see in this slightly edited example, Outlooks boundaries can contain underscores, equal signs and full stops.
I don't know since what version Outlook does use those special characters, but at least in Outlook 2016 those characters are being used according to rfc1341.

The encapsulation boundary MUST NOT appear inside any of the encapsulated parts.

NOTE: Because encapsulation boundaries must not appear in
the body parts being encapsulated, a user agent must
exercise care to choose a unique boundary. The boundary in
the example above could have been the result of an algorithm
designed to produce boundaries with a very low probability
of already existing in the data to be encapsulated without
having to prescan the data. Alternate algorithms might
result in more 'readable' boundaries for a recipient with an
old user agent, but would require more attention to the
possibility that the boundary might appear in the
encapsulated part. The simplest boundary possible is
something like "---", with a closing boundary of "-----".

So Outlook chooses a boundary which has close to zero probability of appearing inside any part of the message.