Removed the ability to continue when the SSL certificate is invalid if MKNetworkOperation.shouldContinueWithInvalidCertificate is set to NO

When shouldContinueWithInvalidCertificate is set to NO the library
should cancel the request if SecTrustEvaluate returns
kSecTrustResultRecoverableTrustFailure.
Merge pull request #341 from tclayson/master

Fix to issue where request is continued with an expired SSL certificate