multipart/form-data work improper when only one form field used.

[Eric-Guo]

Can confirm if I using two form field, it works no problem.

[sferik]

This was resolved by the streaming rewrite in #12.

The old 1.x code built the multipart body by string concatenation, which had an off-by-one boundary issue with a single field. The 2.0 rewrite replaced this with a CompositeIO-based approach that flat_maps a boundary glue before each part and appends a closing boundary.