spec for time_travel_to exception handling

simple spec to make sure an exception is caught when a non Date object
is passed to time_travel_to

spec/helpers/main_helper_spec.rb

@@ -2,10 +2,18 @@
 
 describe TimecopConsole::MainHelper do
   describe "#time_travel_to" do
-    let(:date) { Date.parse('Dec 12, 1950') }
+    context "when a Date object is passed" do
+      let(:date) { Date.parse('Dec 12, 1950') }
 
-    it 'should return a link with timecop console route' do
-      helper.time_travel_to(date).should == "<a data-method=\"post\" href=\"/timecop_console/update?timecop%5Bcurrent_time%281i%29%5D=1950&amp;timecop%5Bcurrent_time%282i%29%5D=12&amp;timecop%5Bcurrent_time%283i%29%5D=12&amp;timecop%5Bcurrent_time%284i%29%5D=12&amp;timecop%5Bcurrent_time%285i%29%5D=0\" rel=\"nofollow\">December 12, 1950</a>"
+      it 'should return a link with timecop console route' do
+        helper.time_travel_to(date).should == "<a data-method=\"post\" href=\"/timecop_console/update?timecop%5Bcurrent_time%281i%29%5D=1950&amp;timecop%5Bcurrent_time%282i%29%5D=12&amp;timecop%5Bcurrent_time%283i%29%5D=12&amp;timecop%5Bcurrent_time%284i%29%5D=12&amp;timecop%5Bcurrent_time%285i%29%5D=0\" rel=\"nofollow\">December 12, 1950</a>"
+      end
+    end
+    context "when a string is passed" do
+      let(:date) { "12-12-1950" }
+      it "should raise an ArgumentError" do
+        expect { helper.time_travel_to(date) }.to raise_error(ArgumentError, "Argument must be a Date object")
+      end
     end
   end
 end