Don't set a default argument as a mutable type

When you run this a second time, `headers` will be the value they were last time you ran it. That seems bad.

http://docs.python-guide.org/en/latest/writing/gotchas/

ckan/lib/mailer.py

@@ -28,7 +28,11 @@ class MailerException(Exception):
 
 def _mail_recipient(recipient_name, recipient_email,
                     sender_name, sender_url, subject,
-                    body, headers={}):
+                    body, headers=None):
+
+    if not headers:
+        headers = {}
+
     mail_from = config.get('smtp.mail_from')
     msg = MIMEText(body.encode('utf-8'), 'plain', 'utf-8')
     for k, v in headers.items():