Fix incorrect multipart boundaries for plain HTTP

[thinkgruen]

Closes #616

Currently the same static value for boundary is set, but it should be different depending on where in the request it is located (beginning, as a separator and at the end).

To note:

• I removed the last newline, because it seems unnecessary according to the standard, so the content-length changed by one or two, depending on whether or not the fixture was trimmed previously.