Upgrade HtmlUnit from 2.x to 3.x

[timja-bot]

This is an autogenerated PR for updating HtmlUnit

It follows the migration guide

Migration steps for cases the script can't handle

This migration currently can't handle HtmlOption changes properly.
If it fails to compile on HtmlOption then revert the specific case:

--- a/src/test/java/hudson/plugins/emailext/ExtendedEmailPublisherDescriptorTest.java
+++ b/src/test/java/hudson/plugins/emailext/ExtendedEmailPublisherDescriptorTest.java
@@ -114,7 +114,7 @@ public class ExtendedEmailPublisherDescriptorTest {
      assertEquals(
              "Plain text should be selected by default",
              "text/plain",
-                contentType.getSelectedOptions().get(0).getValue());
+                contentType.getSelectedOptions().get(0).getValueAttribute());

Ping @timja if any questions

script source

[jglick]

Done in #250.