title: mysql enum is not case sensitive it match the first one without case-sensivity
body: <p>I have <code>enum</code> field that contains <code>lowercase</code> and <code>uppercase</code> of a same letter,
  when I try to update a row and change the value it doesn’t work.</p>
  
  <p>this is the way how reproduce the problem:</p>
  
  <pre><code>CREATE TABLE `mytable` (
    `id` bigint(20) NOT NULL,
    `name` varchar(100) NOT NULL,
    `strategy` enum('g','G','r','R') NOT NULL DEFAULT 'g'
  ) ENGINE=InnoDB;
  
  INSERT INTO `mytable` VALUES(1,'test','g');
  </code></pre>
  
  <p>now when I try to change <code>strategy</code> from <code>g</code> to <code>G</code> it doesn't work:</p>
  
  <pre><code>UPDATE `mytable` SET `strategy`='G' WHERE id=1;
  </code></pre>
  
  <p>it returnes:</p>
  
  <pre><code>Query OK, 0 rows affected (0.00 sec)
  Rows matched: 1  Changed: 0  Warnings: 0
  </code></pre>
  
  <p>I use <code>MySQL 5.5</code>, please help me</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>as mentiond @farshad in his comment,
  It use the first match, if I change the order of <code>enum</code> and use <code>'G','g',...</code> it will always use <code>G</code> and you can not change it back to <code>g</code></p>