title: ruby symbol as key, but can't get value from hash
body: <p>I'm doing some update on other one's code and now I have a hash, it's like:</p>
  
  <pre><code>{"instance_id"=&gt;"74563c459c457b2288568ec0a7779f62", "mem_quota"=&gt;536870912, "disk_quota"=&gt;2147483648, "mem_usage"=&gt;59164.0, "cpu_usage"=&gt;0.1, "disk_usage"=&gt;6336512}
  </code></pre>
  
  <p>and I want to get the value by symbol as a key, for example:   :mem_quota, but failed.</p>
  
  <p>The code is like:</p>
  
  <pre><code>instance[:mem_usage].to_f
  </code></pre>
  
  <p>but it returns nothing. Is there any reason can cause this problem?</p>