Use === for is() and is_not() in contenttests.

The notion of equality used by == in JavaScript is not useful for testing.
Merge pull request #2957 from Ms2ger/is-is_not

Use === for is() and is_not() in contenttests.
 Implement src setter for HTMLIFrameElement #2529
Merge pull request #2956 from Ms2ger/iframe-src

Reload iframes when the src attribute is set; r=jdm