HTMLScriptElementHelpers::is_javascript should compare ASCII case-insensitively

[letharion]

Fix for #3993

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/3202

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[Ms2ger]

@letharion: thanks! I left some comments on critic.

[bors-servo]

saw approval from Ms2ger
at https://github.com/letharion/servo/commit/d84c0fc52a21b317819b2058eb3a735d8d91b1c2

[bors-servo]

merging letharion/servo/Case-insensitive-HTMLScriptElementHelpers-comparision = d84c0fc into auto

[bors-servo]

letharion/servo/Case-insensitive-HTMLScriptElementHelpers-comparision = d84c0fc merged ok, testing candidate = 2964c6a

[bors-servo]

saw approval from Ms2ger
at https://github.com/letharion/servo/commit/d84c0fc52a21b317819b2058eb3a735d8d91b1c2

[bors-servo]

merging letharion/servo/Case-insensitive-HTMLScriptElementHelpers-comparision = d84c0fc into auto

[bors-servo]

No active merge of candidate d84c0fc found, likely manual push to master

[bors-servo]

merging letharion/servo/Case-insensitive-HTMLScriptElementHelpers-comparision = d84c0fc into auto

[bors-servo]

letharion/servo/Case-insensitive-HTMLScriptElementHelpers-comparision = d84c0fc merged ok, testing candidate = 3fafd61

[bors-servo]

all tests pass:
success: http://build.servo.org/builders/linux1/builds/3
success: http://build.servo.org/builders/linux2/builds/3
success: http://build.servo.org/builders/android/builds/70
success: http://build.servo.org/builders/mac1/builds/220
success: http://build.servo.org/builders/mac2/builds/220

[bors-servo]

fast-forwarding master to auto = 3fafd61

[Ms2ger]

Closing to poke bors