title: magento 2 add unique key on multiple columns using installschema
body: <p>Here i am trying to create unique key using <code>installSchema</code> . 
  Array of column names is defined here on which columns combinedly i want to apply unique key</p>
  
  <pre><code>$arrayOfColumnsName = array('column1', 'column2');
  </code></pre>
  
  <p>here defined the addIndex method for creating the unique key </p>
  
  <pre><code>-&gt;addIndex(
              $installer-&gt;getIdxName(
                  $installer-&gt;getTable('table_name'),
                  $arrayOfColumnsName,
                 \Magento\Framework\DB\Adapter\AdapterInterface::INDEX_TYPE_UNIQUE
              ),
              $arrayOfColumnsName,
              ['type' =&gt; \Magento\Framework\DB\Adapter\AdapterInterface::INDEX_TYPE_UNIQUE]
          )
  </code></pre>
  
  <p>This creates unique key for only first column instead of both columns, needs help for the same</p>