fix xsd definition for strict-requirements
merged branch Tobion/strict-req-xsd (PR #7424)

This PR was merged into the 2.2 branch.

Commits
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7ef90d2 fix xsd definition for strict-requirements

Discussion
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fix xsd definition for strict-requirements

see https://groups.google.com/forum/?fromgroups=#!topic/symfony-devs/bvHPI5C4dlY