Remove the `Datum::Json` variant
Remove the `Datum::Json` variant

The `Datum` struct describes the data format which the library and the
server use to communicate. Currently this format is Json and therefore
embeding a variant which holds json data is superfluous.

Background:
RethinkDB sends json encoded data instead of protobuf encoded data if
requested in the query to circumvent slow protobuf libraries.

tomprogrammer/rust-rethinkdb