Update behavior of Document.createElement

[znewman01]

Fixes #4009.

Only lower-case the argument to Document#createElement if it's a HTML document.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3208

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[tetsuharuohzeki]

You should change the commit message to Only lower-case the argument to Document#createElement if it's a HTML document. or more suitable one.

[Ms2ger]

r+

[bors-servo]

saw approval from Ms2ger
at znewman01@a8ce3e3

[bors-servo]

merging znewman01/servo/issue4009 = a8ce3e3 into auto

[bors-servo]

znewman01/servo/issue4009 = a8ce3e3 merged ok, testing candidate = a65bf74

[bors-servo]

No active merge of candidate a8ce3e3 found, likely manual push to master

[bors-servo]

merging znewman01/servo/issue4009 = a8ce3e3 into auto

[bors-servo]

znewman01/servo/issue4009 = a8ce3e3 merged ok, testing candidate = 929671f

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/4
success: http://build.servo.org/builders/linux2/builds/4
success: http://build.servo.org/builders/android/builds/71
success: http://build.servo.org/builders/mac1/builds/221
success: http://build.servo.org/builders/mac2/builds/221

[bors-servo]

fast-forwarding master to auto = 929671f