Fix template_filter decorator to support both usage styles and update docstring; update CHANGES.rst

tests/test_template_filters.py

@@ -0,0 +1,24 @@
+import pytest
+from flask import Flask, render_template_string
+
+def test_template_filter_without_parentheses():
+    app = Flask(__name__)
+
+    @app.template_filter
+    def double(x):
+        return x * 2
+
+    with app.app_context():
+        output = render_template_string("{{ 2 | double }}")
+        assert output == "4"
+
+def test_template_filter_with_parentheses():
+    app = Flask(__name__)
+
+    @app.template_filter()
+    def triple(x):
+        return x * 3
+
+    with app.app_context():
+        output = render_template_string("{{ 3 | triple }}")
+        assert output == "9"