Fix missing boundary when MultipartMixed transport aggregates initial and deferred response

[phughes-scwx]

Adds an additional test to the MultipartMixed transport that triggers aggregation between the initial and a deferred response. Patches the writer to ensure in this case that an extra boundary is written between the two parts.

See the issue details here: #3381 .

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs) (n/a)

[coveralls]

coverage: 59.614% (+0.01%) from 59.601%
when pulling 8b8e52a on phughes-scwx:issue-3381
into 1040b92 on 99designs:master.

[phughes-scwx]

@StevenACoffman @rmennes FYI

[StevenACoffman]

Thanks! Yeah, @defer support could use more thorough testing and verification.