Python SDK: Allow overriding default response headers

[gazpachoking]

When using DatastarResponse headers passed in get merged with the default headers, so there was no way to remove headers included in the default you didn't want (e.g. the x-accel-buffering for nginx.) This also allows sending an extra header with all datastar responses. User provided defaults can be set on the DatastarResponse classes with this change.

DatastarResponse.default_headers = {**SSE_HEADERS, "x-my-extra-header": "foo"}

# ...

    return DatastarResponse()  # user's default headers used
    return DatastarResponse(headers={"x-another": "header"})  # Another header merged in with the defaults

[bencroker]

Merged, thanks!