Raise an error if GetCapabilities requests don't return XML

[geographika]

Currently if a GetCapabilities request doesn't return an XML document it raises cryptic error messages such as:

  File "owslib\feature\wfs200.py", line 164, in _buildMetadata
    for elem in self._capabilities.find(nspath("OperationsMetadata"))[:]:
TypeError: 'NoneType' object is not subscriptable

This pull request checks to see if the server response Content-Type is XML, and if not it raises an exception that includes the URL and response. If the response is HTML it attempts to parse the body of the document to use as the error message. This is particularly useful when working with MapServer which returns errors in text/html format. E.g.

http:///example.org/?service=WFS&request=GetCapabilities&version=2.0.0 responded 
with Content-Type 'text/html': 'msCGILoadMap(): Web application error. CGI variable "map" is not set.'

Currently any Content-Type which has any match/partial match against the following list will be considered as XML: ['text/xml', 'application/xml', 'application/vnd.ogc.wms_xml']

It may be better to just check for xml in case there are types I've missed?

In addition the strip_bom was only previously used for WMS GetCapabilities requests - this change means it is applied to all service types (WFS, SOS, WCS, WMTS).

[coveralls]

Coverage: 59.561% (+0.04%) from 59.523% when pulling 7890a58 on geographika:getcapabilities-errors into 306ceed on geopython:master.

[github-actions]

This Pull Request has been inactive for 90 days. In order to manage maintenance burden, it will be automatically closed in 7 days.

[coveralls]

coverage: 60.189% (+0.5%) from 59.725%
when pulling 58f7a29 on geographika:getcapabilities-errors
into f620405 on geopython:master.

[geographika]

Rebased against main - all tests passing.