Fixed a bug in constant folding of keyword arguments to filter calls.  Thanks noskolo.

--HG--
branch : trunk
diff --git a/jinja2/nodes.py b/jinja2/nodes.py
index 034becf..ec2ed3e 100644
--- a/jinja2/nodes.py
+++ b/jinja2/nodes.py
@@ -464,6 +464,9 @@
     """A key, value pair for keyword arguments where key is a string."""
     fields = ('key', 'value')
 
+    def as_const(self):
+        return self.key, self.value.as_const()
+
 
 class CondExpr(Expr):
     """A conditional expression (inline if expression).  (``{{