added client.set case using force option in order to clarify how to use this option since they are 2 positional hash arguments

Author: joecabezas

.gitignore

@@ -2,3 +2,4 @@
 .bundle
 Gemfile.lock
 coverage/
+.byebug_history

spec/stasche/client_spec.rb

@@ -74,6 +74,11 @@
         error_class = Stasche::Store::KeyAlreadyExistsError
         expect { client.set(foo: 'bar') }.to raise_error(error_class)
       end
+
+      it 'does not throws an exception if forced' do
+        error_class = Stasche::Store::KeyAlreadyExistsError
+        expect { client.set({foo: 'bar'}, {force: true}) }.to_not raise_error
+      end
     end
   end