Fix URI for IE
Fix URI for IE

Summary:
It seems that instead of undefined for regex matches, IE
actually reports an empty string. The spec expects undefined. This fixes
the spec.

Test Plan: Open the specs in IE. Be happy.
Reviewed By: epriestley
Reviewers: tomo, mroch, epriestley
CC: aran, epriestley
Differential Revision: 705

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