WebScreen data detector fix and new feature

There was a big when setting a single `data_detector` on `PM::WebScreen`. I fixed it and write a bunch of tests for attribute setting.

I also added an attribute so that the user can choose to set the `UIWebView.scalesPagesToFit` attribute more easily.
By the way, if we ever decide to change these to `true`, we'll have to change how they're written. Can't just change `false` to `true`.