net: http: Fix HTTP_DATA_FINAL notification

HTTP_DATA_FINAL was incorrectly notified in case Content Length field
was present in the HTTP respone - in such case it was set for every
response fragment, not only the last one.

Fix this by relying on `message_complete` flag instead of
`http_should_keep_alive()` function to determine whether to notify
HTTP_DATA_FINAL or not. As the HTTP parser calls the
`on_message_complete()` callback in either case (response is chunked or
not), this seems to be a more reasonable apporach to determine whether
the fragment is final or not.

Additinally, instead of calling response callback for
`on_body`/`on_message_complete` separately, call it directly from
`http_wait_data()` function, after the parsing round. This fixes the
case when headers were not reported correctly when the provided buffer
was smaller than the total headers length, resulting in corrupted data
being reported to the user.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>