Remove invalid uppercase characters in data-cover attributes

According to the [W3 spec](http://www.w3.org/TR/2011/WD-html5-20110525/elements.html#embedding-custom-non-visible-data-with-the-data-attributes), uppercase characters are only valid if they are
preceded by a hyphen.

Browsers automatically lowercases these invalid characters so these
properties were never found before when Blanket initializes.