Support custom jsonSerializer

[bajtos]

See strongloop/loopback#1650 (comment)

 // a custom JSON serializer function for producing JSON response bodies
 // @param sanitizedData: response data containing only safe properties
 // @param originalError: the original Error object
 jsonSerializer: function(sanitizedData, originalError) {
 if (originalError.name === 'ValidationError') {
 var details = sanitizedData.details || {};
 sanitizedData.issueCount = details.codes && Object.keys(details.codes).length;
 }
 return sanitizedData;
 }