title: Sum 2 hashes attributes with the same key
body: <p>I have 2 hashes, for example:</p>
  
  <pre><code>{'a' =&gt; 30, 'b' =&gt; 14}
  {'a' =&gt; 4, 'b' =&gt; 23, 'c' =&gt; 7}
  </code></pre>
  
  <p>where <code>a</code>, <code>b</code> and <code>c</code> are objects. How can I sum those hashes' keys to get a new hash like:</p>
  
  <pre><code>{'a' =&gt; 34, 'b' =&gt; 37, 'c' =&gt; 7}
  </code></pre>