fix and test for 'not exists' serialization (fixes #178)

Author: jochenberger

json-path/src/main/java/com/jayway/jsonpath/internal/filter/ValueNode.java

@@ -801,7 +801,7 @@ public PathNode asExistsCheck(boolean shouldExist) {
 
         @Override
         public String toString() {
-            return path.toString();
+            return existsCheck && ! shouldExist ? Utils.concat("!" , path.toString()) : path.toString();
         }
 
         public ValueNode evaluate(Predicate.PredicateContext ctx) {

json-path/src/test/java/com/jayway/jsonpath/FilterCompilerTest.java

@@ -84,6 +84,15 @@ public void invalid_filters_does_not_compile() {
         assertInvalidPathException("[?(!'foo')]");
     }
 
+    @Test
+    // issue #178
+    public void compile_and_serialize_not_exists_filter(){
+        Filter compiled = compile("[?(!@.foo)]");
+        String serialized = compiled.toString();
+        assertThat(serialized).isEqualTo("[?(!@['foo'])]");
+    }
+
+
 
     private void assertInvalidPathException(String filter){
         try {