Support for `json: true` equivalent on `send_read_only_transaction` (and other `send_**` endpoints)

[aaroncox]

Currently when using Wharf to access readonly actions, every send_read_only_transaction response is returning both return_value_hex_data (encoded) and return_value_data (decoded). This doubles the size of the response, and I assume causes the server to spend time decoding the data.

Wharf doesn't use the decoded data and instead decodes the data itself in the client after reading the response.

Ideally when making a request, the client making the request would be able to specify json: false (or return_json: false) to specify whether or not the server should include the decoded data.