Support `application/x-www-form-urlencoded` as an additional content
type for `AtomicLevel.ServeHTTP`.

This is the default content type for `curl -X POST`.

With this change, interacting with the HTTP endpoint is a bit more
user friendly:

```
curl -X PUT localhost:8080/log/level -d level=debug
```

Additionally, the unit tests for the HTTP handler are transformed to
a table driven approach.

fixes uber-go#902

These methods do not have need of the atomic level. Turn them into
functions to avoid accidentally manipulating or accessing it.