title: How to ignore case in String.replace
body: <pre><code>string sentence = "We know it contains 'camel' word.";
  // Camel can be in different cases:
  string s1 = "CAMEL";
  string s2 = "CaMEL";
  string s3 = "CAMeL";
  // ...
  string s4 = "Camel";
  // ...
  string s5 = "camel";
  </code></pre>
  
  <p>How to replace 'camel' in sentence with 'horse' despite of <code>string.Replace</code> doesn't support <code>ignoreCase</code> on left string?</p>