Test already installed rubygems in deployment mode

Add test reproducing a crash in deployment mode when rubygems is already installed

tests/test_rubygems.py

@@ -302,6 +302,13 @@ class RubyGemsDeploymentTestCase(RubyGemsTestCase):
         self.install_with_default_rubygems_test(
             path, patches, expected_install_arg_list_list)
 
+    @deployment_fixture({'recipe': {'gems': 'hashie==0.3.1'}})
+    def test_already_installed_rubygems(
+            self, path, patches, buildout, name, options, version):
+        touch(path / 'rubygems/bin/gem')
+        self.deployment_install_test(
+            buildout, name, path, patches, options, version)
+
     @deployment_fixture({'recipe': {'gems': 'sass'}})
     def test_deployment_not_pinned_version_error(
             self, path, patches, buildout, name, options, version):