Fix #2419

[dead-claudia]

Description

Unbreak request bodies without content.

If you do need to pass content, this will provide JSON as the default encoding, and you'll still need to address CORS, but you can still generally send things as plain text.

Motivation and Context

Fixes #2419.

How Has This Been Tested?

Ran tests and added a few new ones to ensure this doesn't break again.

Types of changes

• 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)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md