Add keys function for Map objects

See https://github.com/json-path/JsonPath/issues/439

json-path/src/main/java/com/jayway/jsonpath/internal/function/PathFunctionFactory.java

@@ -2,6 +2,7 @@ package com.jayway.jsonpath.internal.function;
 
 import com.jayway.jsonpath.InvalidPathException;
 import com.jayway.jsonpath.internal.function.json.Append;
+import com.jayway.jsonpath.internal.function.json.KeySetFunction;
 import com.jayway.jsonpath.internal.function.numeric.Average;
 import com.jayway.jsonpath.internal.function.numeric.Max;
 import com.jayway.jsonpath.internal.function.numeric.Min;
@@ -44,6 +45,7 @@ public class PathFunctionFactory {
         map.put("length", Length.class);
         map.put("size", Length.class);
         map.put("append", Append.class);
+        map.put("keys", KeySetFunction.class);
 
 
         FUNCTIONS = Collections.unmodifiableMap(map);

json-path/src/main/java/com/jayway/jsonpath/internal/function/json/KeySetFunction.java

@@ -0,0 +1,23 @@
+package com.jayway.jsonpath.internal.function.json;
+
+import com.jayway.jsonpath.internal.EvaluationContext;
+import com.jayway.jsonpath.internal.PathRef;
+import com.jayway.jsonpath.internal.function.Parameter;
+import com.jayway.jsonpath.internal.function.PathFunction;
+
+import java.util.List;
+
+/**
+ * Author: Sergey Saiyan sergey.sova42@gmail.com
+ * Created at 21/02/2018.
+ */
+public class KeySetFunction implements PathFunction {
+
+    @Override
+    public Object invoke(String currentPath, PathRef parent, Object model, EvaluationContext ctx, List<Parameter> parameters) {
+        if (ctx.configuration().jsonProvider().isMap(model)) {
+            return ctx.configuration().jsonProvider().getPropertyKeys(model);
+        }
+        return null;
+    }
+}

json-path/src/test/java/com/jayway/jsonpath/internal/function/KeySetFunctionTest.java

@@ -0,0 +1,24 @@
+package com.jayway.jsonpath.internal.function;
+
+import com.jayway.jsonpath.Configuration;
+import com.jayway.jsonpath.Configurations;
+import org.apache.commons.io.IOUtils;
+import org.junit.Test;
+
+import java.util.Arrays;
+import java.util.HashSet;
+
+/**
+ * Author: Sergey Saiyan sergey.sova42@gmail.com
+ * Created at 21/02/2018.
+ */
+public class KeySetFunctionTest extends BaseFunctionTest {
+
+    private Configuration conf = Configurations.JACKSON_CONFIGURATION;
+
+    @Test
+    public void testKeySet() throws Exception {
+        String json = IOUtils.toString(getClass().getResourceAsStream("/keyset.json"));
+        verifyFunction(conf, "$.data.keys()", json, new HashSet<String>(Arrays.asList("a", "b")));
+    }
+}

json-path/src/test/resources/keyset.json

@@ -0,0 +1,6 @@
+{
+  "data" : {
+    "a" : "a",
+    "b" : "b"
+  }
+}