Update server.py to catch 400 error

This would stop the no header situation so catch it here.

Author: rockystone77

cheroot/server.py

@@ -732,6 +732,12 @@ def parse_request(self):
                 'allowed bytes.',
             )
             return
+        except ValueError:
+            self.simple_response(
+                '400 Bad request',
+                'Invalid Content Header.',
+            )
+            return
         else:
             if not success:
                 return