fix(http): Don't set content-type w/ form-data

[jnv]

Description

Related to #230 (but not root cause).

When I want to send body with multipart/form-data encoding to Mailgun, I am getting back server error "Missing content boundary". Upon closer inspection, the boundary directive is missing in the Content-Type header. It is normally automatically read from FormData, but since content-type is set explicitly, it's not overwritten.

Thanks to @lukas-valenta for helping me to solve this issue.

Motivation and Context

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have updated the documentation accordingly. If you are changing code related to user secrets you need to really make sure that security documentation is correct.
• [x] I have read the CONTRIBUTING document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[jnv]

@lukas-valenta I have recreated the PR post-release, as discussed.