Add "exec" action to allow execution of arbitrary commands with the app's environment.

spec/helper_spec.rb

@@ -0,0 +1,18 @@
+require "spec_helper"
+
+describe "spec helpers" do
+  describe "#preserving_env" do
+    after { ENV.delete "FOO" }
+    
+    it "should remove added environment vars" do
+      preserving_env { ENV["FOO"] = "baz" }
+      ENV["FOO"].should == nil
+    end
+    
+    it "should reset modified environment vars" do
+      ENV["FOO"] = "bar"
+      preserving_env { ENV["FOO"] = "baz"}
+      ENV["FOO"].should == "bar"
+    end
+  end
+end