send data as a JSON String to fix tests

[davidmartos96]

Fixes tests with the Rust core sync implementation. Not sure if this is needed, or if the Rust part needs to be aware of the data possibly coming as a JSON object instead of String.

[simolus3]

Thanks! The Rust core implementation doesn't have to be aware of directly-encoded data objects (that's a deprecated representation, our current SDKs always request the sync service to encode data). So fixing the tests is correct here.