Allow helper function change() to return request.responseText on request.status === 301

[Rudolf-May]

The R package leaflet.extras2 by trafficonese (https://github.com/trafficonese/leaflet.extras2) makes use of the leaflet.wms plugin.
When the package is used within the context of a shiny-server behind an apache proxy the shiny-server returns a response status code of "301 moved permanently". Because of this I the desired featureinfo is not available in this scenario.
The package works perfectly as it should when the shiny-server is adressed directly without an apache proxy.
Would it be OK if you could add the request.status===301 to the if-condition on line 484 of the source code?

Best regards
Rudolf