Fix testserver to return a correct MIME-type header.

Originally, testserver was sending "Content-type: text/html" header regardless
of a file extension, due to a bug in TestPageHandler.GetMIMETypeFromName.

Patch by Yuta Kitamura <yutak@google.com>.
Original code review: <http://codereview.chromium.org/100357>

Review URL: http://codereview.chromium.org/115058

git-svn-id: http://src.chromium.org/svn/trunk/src/net/tools/testserver@15510 4ff67af0-8c30-449e-8e8b-ad334ec8d88c
diff --git a/testserver.py b/testserver.py
index 9ab77a8..a27b534 100644
--- a/testserver.py
+++ b/testserver.py
@@ -150,7 +150,8 @@
       # no extension.
       return self._default_mime_type
 
-    return self._mime_types.get(extension, self._default_mime_type)
+    # extension starts with a dot, so we need to remove it
+    return self._mime_types.get(extension[1:], self._default_mime_type)
 
   def KillHandler(self):
     """This request handler kills the server, for use when we're done"