Adds unit test checking the key stored.

Signed-off-by: Kouhei Maeda <mkouhei@palmtb.net>

spec/mtodos_spec.rb

@@ -30,4 +30,10 @@
     cli = Mtodos::Client.new(UDD)
     expect(File.exist?('mtodos.cache')).to eq(true)
   end
+
+  it 'store the key in cache after retrieve' do
+    cli = Mtodos::Client.new(UDD)
+    cli.retrieve
+    expect(cli.is_sent?('rc_std_ae0b0e7487e87af44c1b78efbbec037c')).to eq(true)
+  end
 end