Attach updated endpoint signature to inner

[mjpieters]

Not all endpoints accept a __signature__ attribute, nor should the cache decorator modify the decorated endpoint. Attach the signature to the returned inner function instead.

While here, refactor the signature updating code, and extract it to a separate function.

Fixes #115

[mjpieters]

Note: there is one edge-case the decorator isn't handling at the moment: if the decorated endpoint uses the names request or response but expects these to be a different type:

@app.get("/foo/{request}/{response}")
def foo(request: int, response: str) -> None:
    pass

This is perfectly valid, but the cache decorator will then fail with a ValueError: duplicate parameter name: 'request' or ValueError: duplicate parameter name: 'response' exception.

To make that work, you'd have to generate unique names for the request and response parameters. That's well outside the scope of this PR however.

[mjpieters]

Another issue is that the decorator assumes the request and response parameters always are named 'request' and 'response' when popping these from the copy_kwargs dict.

[long2ice]

Thanks!