Add support for proxy method and body

Add support for setting proxy method and proxy body.
We found this useful when using nginx to rewrite our requests.
Usage example:

```
nginx::resource::vhost { $host:
    ensure   => present,
    proxy  => $destination,
    proxy_method => 'POST',
    proxy_set_body => 'hello world'
}
```

I wasn't sure if and where to add method-specific tests for this ;)

(fyi: The comment in `manifests/resource/location.pp` looks a bit out of place, but it's because the others aren't aligned correctly. I'll add a separate MR after this to fix them.)