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.
This is where the bug was. We weren't converting the symbol if there was only one passed to the initializer.