Override dict.copy in HTTPHeaders to return the correct type.

Closes #519.

diff --git a/tornado/httputil.py b/tornado/httputil.py
index 7d884e28bcba23018ad4c962199ef4643660c8a6..2cbf677bfcad0d87aa424ec94d1210f3c31dc306 100644 (file)
--- a/tornado/httputil.py
+++ b/tornado/httputil.py
@@ -154,6 +154,10 @@ class HTTPHeaders(dict):
         for k, v in dict(*args, **kwargs).iteritems():
             self[k] = v
 
+    def copy(self):
+        # default implementation returns dict(self), not the subclass
+        return HTTPHeaders(self)
+
     _NORMALIZED_HEADER_RE = re.compile(r'^[A-Z0-9][a-z0-9]*(-[A-Z0-9][a-z0-9]*)*$')
     _normalized_headers = {}