Defer to GetAttribute in HasAttribute.

The semantics of has_attribute aren't anywhere close to the ones expected for
Element#hasAttribute, and it fails an assertion when passed non-lower-case
names.
auto merge of #3896 : Ms2ger/servo/attr-crash, r=jdm

The semantics of has_attribute aren't anywhere close to the ones expected for
Element#hasAttribute, and it fails an assertion when passed non-lower-case
names.