addresses:
rdar://problem/26385078 Data() really needs a init(length:) and replaceBytes(in:withBytes:)
rdar://problem/26508250 Add more specific init method to Data to indicate keeping a ref type
Initialising with a base64 encoded string was using Base64EncodingOptions rather than Base64DecodingOptions - this means strings with unknown characters cannot be decoded properly as the .ignoreUnknownCharacters option is unavailable