Block API: Validation should be case insensitive

[aduth]

The isEquivalentHTML function currently returns false for the following comparison:

isEquivalentHTML( '<DIV CLASS="foo"></DIV>', '<div class="foo"></div>' );
// false

This should return true, as both the tag name and attribute name should be considered case-insensitive.

References:

Tags contain a tag name, giving the element's name. HTML elements all have names that only use ASCII alphanumerics. In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive.

https://html.spec.whatwg.org/multipage/syntax.html#elements-2

In the HTML syntax, attribute names, even those for foreign elements, may be written with any mix of ASCII lower and ASCII upper alphas. [...] There must never be two or more attributes on the same start tag whose names are an ASCII case-insensitive match for each other.

https://html.spec.whatwg.org/multipage/syntax.html#attributes-2