Changed request metadata to contain only strings
Changed request metadata to contain only strings

- Analytics.log(metadata:) was failing to convert the metadata JSON to string, so hopefully this fixes it

ibm-bluemix-mobile-services/bms-clientsdk-swift-core,ibm-bluemix-mobile-services/bms-clientsdk-swift-core,ibm-bluemix-mobile-services/bms-clientsdk-swift-core