* Fix Multipart consuming payload before header checks
What
--
Split up logic in the constructor into two functions:
- **from_boundary:** build Multipart from boundary and stream
- **from_error:** build Multipart for MultipartError
Also we make the `boundary`, `from_boundary`, `from_error` methods public within the crate so that we can use them in the extractor.
The extractor is then able to perform header checks and only consume the
payload if the checks pass.
* Add tests
* Add payload consumption test
Co-authored-by: Rob Ede <robjtede@icloud.com>
