title: IIS URL Rewrite: formaction custom tag-attribute ignored
body: <p>I have a simple outbound URL rewrite rule on my IIS ASP.Net core 2.2 webserver (IIS version 8.5.9600.16384):</p>
  
  <pre><code>&lt;outboundRules&gt;
      &lt;clear /&gt;
      &lt;rule name="CustomTags" preCondition="" stopProcessing="true"&gt;
          &lt;match filterByTags="CustomTags" customTags="Misc" pattern="(.*)" /&gt;
          &lt;conditions logicalGrouping="MatchAll" trackAllCaptures="true" /&gt;
          &lt;action type="Rewrite" value="Hello world!" /&gt;
      &lt;/rule&gt;
      &lt;customTags&gt;
          &lt;tags name="Misc"&gt;
              &lt;tag name="button" attribute="formaction" /&gt;
              &lt;tag name="input" attribute="formaction" /&gt;
              &lt;tag name="input" attribute="type" /&gt;
              &lt;tag name="div" attribute="class" /&gt;
          &lt;/tags&gt;
      &lt;/customTags&gt;
  &lt;/outboundRules&gt;
  </code></pre>
  
  <p>Using the rule on the <code>html</code> code:</p>
  
  <pre><code>&lt;div class="card"&gt;&lt;/div&gt;
  &lt;input type="submit" value="My button" formaction="/path-to-some-page" /&gt;
  &lt;button class="btn" type="button" formaction="/path-to-some-page"&gt;My button&lt;/button&gt;
  </code></pre>
  
  <p>I get the following rewrites:</p>
  
  <pre><code>&lt;div class="Hello world!"&gt;&lt;/div&gt;
  &lt;input type="Hello world!" value="My button" formaction="/path-to-some-page" /&gt;
  &lt;button class="btn" type="button" formaction="/path-to-some-page"&gt;My button&lt;/button&gt;
  </code></pre>
  
  <p>All my custom tags except the ones based on the attribute <code>formaction</code> is rewritten as expected. </p>
  
  <p><strong>Question:</strong> Why is the <code>formaction</code> attribute ignored/protected?</p>