Initialize HTTParty requests with an URI object and a String.

Change `URI.parse(uri)` to `URI(uri)` so that HTTParty consuming code can make requests with:

```ruby
uri = URI('http://www.google.com/')
HTTParty.get(uri)
```

as opposed to 

```ruby
uri = URI('http://www.google.com/')
HTTParty.get(uri.to_s)
```

without throwing and invalid URI exception.