update missing check spec to mock RACK_ENV

The spec result has a precondition in the expectations.
The RACK_ENV must be `development` and causes the test to
fail based on test execution order in scenarios where a
previous test set a different expectation in the env.

spec/api/json_rpc_spec.rb

@@ -256,6 +256,10 @@ RSpec.describe "Metasploit's json-rpc" do
     end
 
     context 'when the module does not support a check method' do
+      before do
+        mock_rack_env('development')
+      end
+
       let(:module_name) { 'scanner/http/title' }
 
       it 'returns successful job results' do