ws_upgrade doesn't alter keepalive value

Given the following code snippet:

```erlang
gun:open(Host, Port, #{})
gun:ws_upgrade(Pid, Url, [], #{keepalive=> infinity})
```

The keepalive continues to use the default value, and a keepalive is sent after 5 seconds.

The following correctly sets the keepalive value:

```erlang
gun:open(Host, Port, #{ws_opts => #{keepalive => infinity}})
gun:ws_upgrade(Pid, Url, [], #{})
```

