Bug 1411281 - Add equality test for webdriver.Session r=jgraham

When comparing two instances of webdriver.Session we want to first
check that there is a current session, then the type of the object
to compare with to make sure the "session_id" attribute is present,
then finally we compare the session IDs.

MozReview-Commit-ID: 6Ch4Uy2MEhB

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extra : rebase_source : 252fe31680c57831b122d2bb53fe133b32c322eb