Allow to retrieve all variable names of a Scope (#496)

* Add a test file to test GetKeys of the Scope class

* Add a GetKeys method

* Change method name to camelCase

* Fix test

pebble/src/main/java/com/mitchellbosecke/pebble/template/Scope.java

@@ -9,6 +9,7 @@
 package com.mitchellbosecke.pebble.template;
 
 import java.util.HashMap;
+import java.util.Set;
 import java.util.Map;
 
 /**
@@ -96,4 +97,13 @@ public boolean containsKey(String key) {
   public boolean isLocal() {
     return this.local;
   }
+
+  /**
+   * Returns keys of all the variables at this scope.
+   *
+   * @return A set of keys
+   */
+  public Set<String> getKeys(){
+    return backingMap.keySet();
+  }
 }

pebble/src/test/java/com/mitchellbosecke/pebble/ScopeTest.java

@@ -0,0 +1,28 @@
+package com.mitchellbosecke.pebble;
+
+import com.mitchellbosecke.pebble.template.Scope;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class ScopeTest {
+
+    @Test
+    void testGetKeys() {
+        Map<String, Object> map = new HashMap<>();
+        map.putIfAbsent("key1", new String("value1"));
+        map.putIfAbsent("key2", new String("value2"));
+
+        Scope scope = new Scope(map, false);
+        Set<String> expected = new HashSet<>();
+        expected.add("key1");
+        expected.add("key2");
+
+        assertEquals(expected, scope.getKeys());
+    }
+}