Implement ES6 compact-style multi-select-hash.

Author: springcomp

src/jmespath.net.parser/JmesPathParser.cs

@@ -181,6 +181,12 @@ void OnHashWildcardProjection() =>
         void AddMultiSelectHashExpression() => generator_.AddMultiSelectHashExpression();
         void PopMultiSelectHash() => generator_.PopMultiSelectHash();
 
+		void AddCompactHashExpression(Token token)
+		{
+			generator_.OnIdentifier((string)token.Value);
+			generator_.AddMultiSelectHashExpression();
+		}
+
         #endregion
 
         #region multi_select_list

src/jmespath.net.parser/JmesPathParser.y

@@ -246,7 +246,28 @@ hash_wildcard		: T_STAR
 					}
 					;
 
-multi_select_hash	: T_LBRACE keyval_expressions T_RBRACE
+multi_select_hash	: multi_select_hash_key
+					| multi_select_hash_keyval
+					;
+
+multi_select_hash_key
+					: T_LBRACE key_expressions T_RBRACE
+					{
+						PopMultiSelectHash();
+					};
+
+key_expressions		: identifier
+					{
+						PushMultiSelectHash();
+						AddCompactHashExpression($1.Token);
+					}
+					|key_expressions T_COMMA identifier
+					{
+						AddCompactHashExpression($3.Token);
+					};
+
+multi_select_hash_keyval
+					: T_LBRACE keyval_expressions T_RBRACE
 					{
 						PopMultiSelectHash();
 					};

tests/jmespathnet.tests/Parser/MultiSelectHashTest.cs

@@ -29,5 +29,11 @@ public void ParseMultiSelectHash_Compliance()
         {
             Assert("missing.{foo: bar}", "[]", "null");
         }
+        [Fact]
+        public void ParseMultiSelectHash_Compact()
+        {
+            Assert("{foo: foo, baz: baz}", "{\"foo\": \"bar\", \"baz\": \"qux\"}", "{\"foo\":\"bar\",\"baz\":\"qux\"}");
+            Assert("{foo, baz}", "{\"foo\": \"bar\", \"baz\": \"qux\"}", "{\"foo\":\"bar\",\"baz\":\"qux\"}");
+        }
     }
 }