double if statement (#707)

* Migrated `%` string formating

* double if statement

combined double 'if' to a single 'if' with 'and'

* Revert "Fix "Prefer `format()` over string interpolation operator" issue"

sanic/cookies.py

@@ -116,8 +116,7 @@ class Cookie(dict):
                     ))
                 except AttributeError:
                     output.append('%s=%s' % (self._keys[key], value))
-            elif key in self._flags:
-                if self[key]:
+            elif key in self._flags and self[key]:
                 output.append(self._keys[key])
             else:
                 output.append('%s=%s' % (self._keys[key], value))