Describe the bug
I'm using a custom error for my server functions and I'm setting type Encoder = leptos::server_fn::codec::JsonEncoding in my impl FromServerFnError. I would expect this to set the content-type: application/json header like leptos does for successful API responses, but the header is not set at all. I noticed that this also impacts the built-in ServerFnError -- I would expect it to set the header to text/plain, but it's missing.

Leptos Dependencies

Leptos 0.8.6 and the latest main branch. Example repro: spencewenski@3cd8c37

To Reproduce
Steps to reproduce the behavior:

1. Update a server function to return an error. In the above repro, clear_server_count is updated.
2. Run the counter_isomorphic example in in the above repro.
3. Click the Clear button
4. Observe that the response does not include the content-type.

Screenshots

Next Steps

• I will make a PR
• I would like to make a PR, but need help getting started
• I want someone else to take the time to fix this
• This is a low priority for me and is just shared for your information

Additional context
Add any other context about the problem here.