Add ability to set custom boundary for multipart entity

[siriniok]

Why

There is a lot of weird APIs in the wild and to use some of them we need to know our MultiPart boundary. The easiest way to obtain this knowledge is to set this boundary manually ourselves. So please accept my small improvement that allows to do just that.

Tests

Can't run lein all test :all for a reason described in #618. It doesn't seem to be related to my changes anyhow though.

However, lein test clj-http.test.multipart-test runs without errors:

$ lein test clj-http.test.multipart-test

Ran 3 tests containing 55 assertions.
0 failures, 0 errors.