title: Ruby: Get all keys in a hash (including sub keys)
body: <p>let's have this hash:</p>
  
  <pre><code>hash = {"a" =&gt; 1, "b" =&gt; {"c" =&gt; 3}}
  hash.get_all_keys 
  =&gt; ["a", "b", "c"]
  </code></pre>
  
  <p>how can i get all keys since <code>hash.keys</code> returns just <code>["a", "b"]</code></p>