Headers not working

This command should not result in this error, as I also tried without the header and the URL is correctly parsed, only when adding the header the command does not work anymore.

```shell
> oha -H "foo: bar" http://localhost:8080/
error: The following required arguments were not provided:

USAGE:
oha [FLAGS] [OPTIONS]
```

And the headers `-H` resides in the `OPTIONS` section, so this should be fine.
`-H "foo: bar"` was taken from the example in the help.

oha version 0.4.7