Look up proxyType case insensitively

When deserializing, be kind to folks who are using enums
in the local end and have sent the names of the proxyType
in upper case.

Closes #803

Author: shs96c

webdriver-spec.html

@@ -419,6 +419,7 @@ <h3>Dependencies</h3>
   in the Infra standard: [[!INFRA]]
    <ul>
     <!-- ASCII lower alpha --> <li><dfn><a href=https://infra.spec.whatwg.org/#ascii-lower-alpha>ASCII lower alpha</a></dfn>
+    <!-- ASCII lowercase --> <li><dfn><a href=https://infra.spec.whatwg.org/#ascii-lowercase>ASCII lowercase</a></dfn>
    </ul>
 
  <dt>Page Visibility
@@ -2022,6 +2023,9 @@ <h3>Proxy</h3>
    <li><p>Let <var>value</var> be the result of <a>getting a
     property</a> named <var>name</var> from <var>parameter</var>.
 
+   <li><p>Let <var>key</var> be the result of
+    <a data-lt="ASCII lowercase">lowercasing</a> <var>key</var>.
+
    <li><p>If there is no matching <code>key</code> for <var>key</var>
     in the <a>proxy configuration</a> table return an <a>error</a>
     with <a>error code</a> <a>invalid argument</a>.