Bugfix: Multipart Content-Length

[MGatner]

Description
Following RFC 7230, CURL requests currently set the Content-Length header to 0 for every POST and PUT request, to make sure it is present. This is overwritten by the actual length for application/x-www-form-urlencoded requests, but left in place for multipart/form-data which causes some servers to close the connection early (since 0 bytes are already done).

This PR makes the 0 setting conditional on the presence of multipart data, so the native CURL client can set the actual Content-Length automatically.

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• n/a User guide updated
• Conforms to style guide