If use_legacy_notifications is True, set to 'true'

Andreas Larsson from Pingdom Support says:

```
I'm afraid I can't really see anything wrong with [your request] Allard, 

A request that looks like this:

api.pingdom.com/api/2.0/checks/<checkid>?sendnotificationwhendown=<minutes>&use_legacy_notifications=true

Will work to modify it, might it be that you should change use_legacy_notifications=1 to use_legacy_notifications=true instead?

Can't really troubleshoot third party tools like this but it's what I suspect at least.
```

And yes, it seems like requests is formulating a form POST, and turning `True` values into the integer `1`. This is not accepted by Pingdom, so handle it here.

It might be generic to all boolean values, but I don't really know. It is very intuitive to set these values to `True`, not `"true"`, so I'd like to translate this one.