Empty POST requests to an API have the content-type header removed

[geoffrich]

Describe the bug

Empty POST requests to the API of an Azure Static Web App have the content-type header removed.

To Reproduce

repro

• create an Azure Function in your SWA that handles POST requests
• inspect the content-type header received by that function (either by logging or by returning the value)
• if the POST request has no body, the content-type header is not present. This is despite the content-type header being present on the original request from the browser

live demo

Click "submit" on the empty form. The response from the Azure function shows that no content-type header was passed to the function handler. However, if you inspect the request sent from the browser, the content-type header was sent -- so something is removing the header before it gets to the Azure function.

Submitting the second form (which has a form body) does not strip the content-type header.

Expected behavior

I expect the Azure function to receive the same content-type header that was sent from the browser.

Device info (if applicable):
n/a

Additional context

I maintain an Azure SWA adapter for SvelteKit and this behavior caused this issue.

This appears to be SWA specific, instead of a more general Azure function problem. I deployed the same Azure function code to a standalone function and it did not remove the content-type header.