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
738009399d