map of trailer fields in which all the keys are in lowercase, regardless of the case they had at the
 protocol level. If there are no trailer fields, yet <a href="#isTrailerFieldsReady()"><code>isTrailerFieldsReady()</code></a> is returning true, the empty map is
 returned.</dd>
<dt>Throws:</dt>
<dd><code><a href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/IllegalStateException.html" title="class or interface in java.lang" class="external-link">IllegalStateException</a></code> - 