title: What is wrong with func NamedExec?
body: <p><strong>go version</strong>:   <code>1.14.6</code><br />
  <strong>sqlx version</strong>: <code>latest</code></p>
  <p><strong>wrong code as follow:</strong></p>
  <pre class="lang-golang prettyprint-override"><code>test := Test{
    Name: &quot;John&quot;,
    Age:  30,
    Id:   &quot;bskdvfjreo018g2c5pqg&quot;,
  }
  // table test has fields: id, name, age
  _, err := sqlxdb.DB.NamedExec(&quot;NSERT INTO test(name,age,id)VALUES(:Name,:Age,:Id)&quot;, &amp;test)
  log.Println(err)
  
  // log as follow:
  
  // 2020/08/07 11:49:15 could not find name Name in &amp;sqlx.Test{Id:&quot;bskdvfjreo018g2c5pqg&quot;, Name:&quot;John&quot;, CreateAt:0, Age:30}
  </code></pre>
  <p>Dose anyone knows whether or not the second parameter of <code>NamedExec</code> must pass value like &quot;:create_at&quot; but not like &quot;:CreateAt&quot; and so one?</p>