INT-2405-maint: polishing tests:

XPathHeaderEnricherTests#notSkippingNullValues() add 'enricher.setDefaultOverwrite(true);' as right behavior for overwriting on existing headers

INT-2405 fix MessageProcessor for <header-value>
1. MethodInvokingHeaderValueMessageProcessor => MessageProcessingHeaderValueMessageProcessor because now he uses MessageProcessor
2. polishing HeaderEnricherParserSupport
3. GroovyHeaderEnricherTests: add check to HeaderValueMessageProcessor for Groovy scripts.
4. build.gradle: add dependency to 'spring-integration-test' for 'spring-integration-groovy'.
5. Fix HeaderEnricherParserSupport like in INT-2188

spring-integration-xml/src/test/java/org/springframework/integration/xml/transformer/XPathHeaderEnricherTests.java

@@ -72,6 +72,7 @@ public class XPathHeaderEnricherTests {
 		String docAsString = "<root><elementOne>1</elementOne></root>";
 		XPathHeaderEnricher enricher = new XPathHeaderEnricher(expressionMap);
 		enricher.setShouldSkipNulls(false);
+		enricher.setDefaultOverwrite(true);
 		Message<?> result = enricher.transform(MessageBuilder.withPayload(docAsString).setHeader("two", "x").build());
 		MessageHeaders headers = result.getHeaders();
 		assertNull(headers.get("two"));