Uppercase element.tagName for HTML elements only

[mbrubeck]

Fixes #3146

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/2443

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[jdm]

 3:16.24 TEST_START: Thread-TestrunnerManager-3 /dom/nodes/Element-tagName.html

3:16.46 TEST_END: Thread-TestrunnerManager-3 Harness OK. Subtests passed 1/3. Unexpected 1

tagName should not upper-case for SVG elements in HTML documents.

-----------------------------------------------------------------

Expected FAIL, got PASS

 2:23.05 TEST_START: Thread-TestrunnerManager-3 /dom/nodes/Document-createElementNS.html

2:23.28 TEST_END: Thread-TestrunnerManager-3 Harness OK. Subtests passed 31/39. Unexpected 3

Lower-case non-HTML element without a prefix

--------------------------------------------

Expected FAIL, got PASS

Lower-case non-HTML element with a prefix

-----------------------------------------

Expected FAIL, got PASS

Upper-case non-HTML element with a prefix

-----------------------------------------

Expected FAIL, got PASS

[jdm]

failures:
/home/travis/build/servo/servo/src/test/content/test_document_body.html
/home/travis/build/servo/servo/src/test/content/test_document_head.html

[Manishearth]

#3145 merged, needs an update-wpt

[mbrubeck]

The test failure appears to be an unrelated intermittent failure; filed #3180.