net: lib: http: Correct http_client edge-case behavior

Correct various small edge-case behaviors that have been accidentally
introduced in the http_client.

- http_client_req no longer incorrectly returns -ETIMEDOUT on NULL HTTP
  resonse. -ETIMEDOUT is now only returned when the underlying TLS
  socket times out.
- http_client_req now returns -ECONRESET upon incomplete (but non-NULL)
  HTTP response. The request callback is no longer called in this case
  (as with any other error state).
- http_wait_data has been refactored slightly to increase clarity.

Signed-off-by: Georges Oates_Larsen <georges.larsen@nordicsemi.no>