Standardise Data Mappings in Seldon Wrapper

[axsaucedo]

Currently the data conversions from Python data structures into SeldonMessage components (i.e. data, strData, jsonData or binData) are not fully standardised. For example, the only way to retunr a jsonData atm is by sending jsonData as input. A fix would be to set the data to be converted into Dict by default. But it would also be important to consider cases such as when we return "strData" or "binData", as these can be valid jsonData as well.