fix: Fix for test failing on JDK 11/12 #967

Author: Ronald Holshausen

consumer/pact-jvm-consumer/src/test/groovy/au/com/dius/pact/consumer/dsl/DslPartSpec.groovy

@@ -6,7 +6,7 @@ import spock.lang.Unroll
 class DslPartSpec extends Specification {
 
   @SuppressWarnings('MethodCount')
-  private final DslPart subject = new DslPart('', '') {
+  private static final DslPart subject = new DslPart('', '') {
 
     @Override
     protected void putObject(DslPart object) { }