title: Liquibase "modifyDataType" tag always makes the column nullable
body: <p>When I do-</p>
  <pre class="lang-xml prettyprint-override"><code>&lt;modifyDataType 
      tableName=&quot;EMCostUnit&quot; 
      columnName=&quot;GUID&quot; 
      newDataType=&quot;VARCHAR(50)&quot;/&gt;
  </code></pre>
  <p>In liquibase, it always resets the nullable constraints. Is it possible to add some parameter so it didn't remove the constraints, because now after each <code>modifyDataType</code>, I need additional <code>changeSet</code> to add back not null constraints.</p>