Allow streamable response as response_class to work

[92dev]

Basically classes such as StreamingResponse in starlette cannot work unless specified directly in the response-type due to this serialization concept (which they always fallback to use JSON)

There is the alternative which I opted out of for now, which is pass a response_serialize: bool parameter throughout the entire flow, it might be better but I wanted to avoid API changes (even through defaults) until any maintainer gives his opinion on the change

example usage:

from fastapi.responses import StreamingResponse

# Here `response_class` is only used for documentation
@app.get('/test_before', response_class=StreamingResponse)
async def before_fix(msg: str):
    async def txt_generator():
        for i in range(10):
            yield f'event: something\ndata: {i}\n\n'
    return StreamingResponse(txt_generator())

# Here `response_class` is actually working
@app.get('/test_after', response_class=StreamingResponse)
async def after_fix(msg: str):
    async def txt_generator():
        for i in range(10):
            yield f'event: something\ndata: {i}\n\n'
    return txt_generator()