title: Issues with Go XML encoding
body: <p>I need to produce the following xml :</p>
  
  <pre><code>&lt;AccessControlList&gt;
      &lt;Grant&gt;
          &lt;Grantee
              xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser"&gt;
              &lt;ID&gt;xxxx-xx&lt;/ID&gt;
              &lt;DisplayName&gt;rahul.khan&lt;/DisplayName&gt;
          &lt;/Grantee&gt;
          &lt;Permission&gt;FULL_CONTROL&lt;/Permission&gt;
      &lt;/Grant&gt;
  &lt;/AccessControlList&gt;
  </code></pre>
  
  <p>My struct is defined like this :</p>
  
  <pre><code>type Grantee struct {
  Xmlns_xsi   string `xml:"xmlns xsi,attr,omitempty"`
  Xsi_type    string `xml:"http://www.w3.org/2001/XMLSchema-instance type,attr,omitempty"`
  ID          string `xml:",omitempty"`
  DisplayName string `xml:",omitempty"`
  </code></pre>
  
  <p>}</p>
  
  <p>However when i marshall this structure the resulting xml document that is produced looks like this :</p>
  
  <pre><code>&lt;AccessControlList&gt;
      &lt;Grant&gt;
          &lt;Grantee
              xmlns:XMLSchema-instance="http://www.w3.org/2001/XMLSchema-instance" XMLSchema-instance:type="CanonicalUser"&gt;
              &lt;ID&gt;xxxx-xx&lt;/ID&gt;
              &lt;DisplayName&gt;rahul.khan&lt;/DisplayName&gt;
          &lt;/Grantee&gt;
          &lt;Permission&gt;FULL_CONTROL&lt;/Permission&gt;
      &lt;/Grant&gt;
  &lt;/AccessControlList&gt;
  </code></pre>
  
  <p>As a result, when the doc is unmarshalled, the type field doesn't seem to be parsed by the aws-go-sdk. </p>
  
  <p>For e.g this is the unmarhsalled output that i need to get </p>
  
  <pre><code>Grantee: {
      DisplayName: "rahul.khan",
      ID: "xxxx-xx",
      Type: "CanonicalUser"
    },
    Permission: "FULL_CONTROL"
  }
  </code></pre>
  
  <p>Instead what I get is this :</p>
  
  <pre><code>  Grantee: {
      DisplayName: "rahul.khan",
      ID: "xxxx-xx"
    },
    Permission: "FULL_CONTROL"
  }
  </code></pre>
  
  <p>The Type attribute seems to be missing from the unmarshalled output. The only difference that I see in the xml document produced by my code and from aws is this line </p>
  
  <p><code>xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="CanonicalUser"
  </code>
  v/s</p>
  
  <p><code>xmlns:XMLSchema-instance="http://www.w3.org/2001/XMLSchema-instance" XMLSchema-instance:type="CanonicalUser"
  </code></p>
  
  <p>Appreciate if someone can help me understand how to fix this issue ?</p>