Update behavior of Document.createElement

Fixes #4009.

Only lower-case the argument to Document#createElement if it's a HTML document.
Lower case the type attribute before checking if it's JS
auto merge of #4021 : letharion/servo/Case-insensitive-HTMLScriptElementHelpers-comparision, r=Ms2ger

Fix for #3993
Implement DOMImplementation.hasFeature

Fixes #4010.
auto merge of #4029 : znewman01/servo/issue4009, r=Ms2ger

Fixes #4009.

Only lower-case the argument to Document#createElement if it's a HTML document.
auto merge of #4027 : znewman01/servo/issue4010, r=Ms2ger

Fixes #4010.

This is my first Servo contribution, so let me know if I missed anything!
Stop including the element during Element.getElementsByClassName.

https://github.com/servo/servo/issues/3995