[MapProxy-dev] typo in attribute of map object [has fix]

[Tobias Wendorff]

Hi there,

I've just dropped a PR: https://github.com/mapproxy/mapproxy/pull/675

As reported in #674 and discussed with the original coder, the commit 
fixes a typo in the attribute name carrying the map object's PID, which 
leads to an uncaught crash.

PTAL.

It is extremely difficult to write a test for this, since the error only 
appears in very specific situations and randomly.

Best regards,
Tobias