Restore support for adding "px" to non-numeric setStyle arguments

Unless you had a specific reason to drop support for this, this just restores the TinyMCE3 behavior of dom.setStyle that would accept a non-numeric string e.g. "32" and convert it to a "32px" as TinyMCE4 currently does for pure numeric values. I also added tests to cover expected behavior of adding px both for a string and a numeric value that doesn't include px already.