capybara-json 
testing ruby: 1.9.3, 2.0.0, 2.1.x, 2.2.x; Capybara: 0.4.x, 1.1.x, 2.0.x
About capybara-json
capybara-json provides the same interface to testing JSON API (both local and remote)
Capybara is an acceptance test framework, and it has no interest with client error(4xx response).
USAGE
require 'capybara/json'
include Capybara::Json
Capybara.current_driver = :rack_test_json
Capybara.app = MyRackApp
post '/', { "this is" => "json" } # POST '/'
json #=> parsed json response
raw_json #=> raw response body
get '/errors/400'
status_code #=> 400
get! '/errors' #=> raise Capybara::Json::Error
get '/errors', {}, { 'header' => '' } # set request headers
response_headers #=> get response headers
Capybara.current_driver = :httpclient_json
Capybara.app_host = 'http://example.com'
post '/', { "this is" => "json" } # POST 'http://example.com/'
json #=> parsed json response
raw_json #=> raw response body
get '/errors/400'
status_code #=> 400
get! '/errors' #=> raise Capybara::Json::Error
get '/errors', {}, { 'header' => '' } # set request headers
response_headers #=> get response headers