Supports /api/dataset/:export-format endpoint

[mrtc0]

This PR is support dataset endpoint /api/dataset/#{format} .

query = '{"type":"native","native":{"query":"SELECT * FROM orders LIMIT 1;"},"database":1}'
client.query_dataset(query: query)
# =>[{"CREATED_AT"=>"2019-02-11T21:40:27.892", "PRODUCT_ID"=>14,"USER_ID"=>1,...}]

This endpoint requires a Content-Type header for application/x-www-form-urlencoded, so I made it possible to set custom headers.

[shimoju]

Thanks!!!