title: Mock Laravel's Config facade to return a value only for a certain key
body: <p>I would like to mock <code>Config::get('specific_key')</code> to return a <code>'specific_value'</code> in my test. So I wrote the following code:</p>
  
  <pre><code>Config::shouldReceive('get')
      -&gt;with('specific_key')
      -&gt;andReturn('specific_value');
  Config::makePartial();
  </code></pre>
  
  <p>This will work: if I add <code>dd(Config::get('specific_key'))</code> I will receive <code>'specific_value'</code>. </p>
  
  <p>However, if I do <code>dd(Config::get('another_key'))</code>, I don't receive any value (I guess because the mock doesn't expect this key as an argument).</p>
  
  <p>So my question is: Is there a way to mock Config's get() method to return a specific value only for an specific key (and return normal value from the configuration file for any other key)?</p>