Enquire about return_value of RunUint8Array

[tpmccallum]

When the vm.RunUint8Array runs in conjunction with a callback [1], we need to decide how the return_value (which is given directly from SSVM execution) is packaged up in readiness for the callback.

Reason being, the callback object is always a JSON string. Therefore the return_value needs to be converted from it's native byte array to Array / String in the JSON and then potentially back again depending on the incoming Content-Type of the callback.

[1] https://github.com/second-state/wasm-joey/blob/master/src/server.js#L407

[tpmccallum]

This is now set to application/octet-stream
https://github.com/second-state/wasm-joey/blob/master/src/server.js#L1402
https://github.com/second-state/wasm-joey/blob/master/src/server.js#L1409