dataset should not accept upper case letter

http://www.w3.org/TR/2012/CR-html5-20121217/dom.html#embedding-custom-non-visible-data-with-the-data-*-attributes
'2.For each content attribute on the element whose first five characters are the string "data-" and whose remaining characters (if any) do not include any characters in the range U+0041 to U+005A (LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z)'