Fix to avoid attribute name clash between Document/HTMLDocument

[brunoabinader]

Use children() instead of traverse_preorder(), and avoid having
GetHead() in both Document and HTMLDocument.

Closes #1465.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/534

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[Ms2ger]

LGTM. @jdm?

[bors-servo]

saw approval from jdm
at https://github.com/brunoabinader/servo/commit/b5eba001efe209bb9f75c7eb61b8affc848bb2e8

[bors-servo]

merging brunoabinader/servo/document-head = b5eba00 into auto

[bors-servo]

brunoabinader/servo/document-head = b5eba00 merged ok, testing candidate = e8b0eae

[bors-servo]

all tests pass:
success: http://servo-buildbot.pub.build.mozilla.org/builders/linux/builds/779
success: http://servo-buildbot.pub.build.mozilla.org/builders/mac/builds/734

[bors-servo]

fast-forwarding master to auto = e8b0eae