Test for XML in response, regardless of mimetype

[geographika]

Fix for #983. application/octet-stream is a very generic mime type which could contain anything, so rather than adding more mime types to the list, a check is made to see if the response contains an xml tag, or is one of the following mime types:

['text/xml', 'application/xml', 'application/vnd.ogc.wms_xml']

@SimonSAMPERE - can you check if this works with the WFS/WMS services that previously were failing?

[SimonSAMPERE]

@geographika it works, well done!