MockHttpServletResponse stores error code specified in sendError as a separate field from statusCode.  When a servlet does not have a method implemented, the default servlet implementation uses sendError.  A developer could also invoke sendError from their own implementation.  In these cases the RestSpecServletValidator would not report a violation of the spec.  This commit fixes #8, and will now report a violation if sendError is used.  (pair [dmasyukov dsiefert kramakrishnan])
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