Set the content type in proxy settings test.

Without this, the htmlunit driver believes we've returned a page of text,
and then can't find elements.

Author: shs96c

java/client/test/org/openqa/selenium/ProxySettingTest.java

@@ -235,6 +235,7 @@ private WebServer createSimpleHttpServer(final String responseHtml) {
       public void handleHttpRequest(
           HttpRequest request, HttpResponse response, HttpControl control) {
         response.charset(Charsets.UTF_8)
+            .header(HttpHeaders.CONTENT_TYPE, "text/html")
             .content(responseHtml)
             .end();
       }