Added more tests to cover adding multiple string values to the keychain and updating strings.
Added more tests to cover adding multiple string values to the keychain and updating strings.

rnewman/SwiftKeychainWrapper,rnewman/SwiftKeychainWrapper,mozilla/SwiftKeychainWrapper,rnewman/SwiftKeychainWrapper,jrendel/SwiftKeychainWrapper,jrendel/SwiftKeychainWrapper,mozilla/SwiftKeychainWrapper,tobiasoleary/SwiftKeychainWrapper,tobiasoleary/SwiftKeychainWrapper,mozilla/SwiftKeychainWrapper