Basic support for Document::contentType

Spec:
http://dom.spec.whatwg.org/#dom-document-contenttype

This is a subtask for #1428, #1510 and #1526.
auto merge of #1525 : brunoabinader/servo/document-contenttype, r=Ms2ger

Spec:
http://dom.spec.whatwg.org/#dom-document-contenttype

This is a subtask for #1428 and #1510.