Fixed an issue where adding items to the root site collection resulted in a 404

[DaleyKD]

This is to help fix #312 .

We were adding a double-slash in the list URL (e.g. - "https://gimmalkyle2.sharepoint.com//lists/MyTest")

Question for:

                // Drop List suffix
                listUrl = listUrl.Substring(0, listUrl.Length - 4);
                return $"{contextUrl}/lists/{listUrl}";

I see we're stripping off "List" from the end using the .Substring(). However, if the language is anything other than English, does that matter (I know it's coming from the OData_ property bag values)?

[codecov-commenter]

Codecov Report

Merging #470 (e4f6f72) into dev (c13436b) will increase coverage by 3.80%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #470      +/-   ##
==========================================
+ Coverage   75.32%   79.13%   +3.80%     
==========================================
  Files         257      333      +76     
  Lines       19708    22834    +3126     
==========================================
+ Hits        14846    18070    +3224     
+ Misses       4862     4764      -98     

Impacted Files	Coverage Δ
...e.Auth/Public/InteractiveAuthenticationProvider.cs	70.00% <100.00%> (+3.80%)	⬆️
...del/SharePoint/Core/Internal/FieldLocationValue.cs	0.00% <0.00%> (-35.49%)	⬇️
...h/Public/UsernamePasswordAuthenticationProvider.cs	68.23% <0.00%> (-25.89%)	⬇️
...P.Core/QueryModel/Query/BaseDataModelExtensions.cs	38.88% <0.00%> (-23.07%)	⬇️
.../Model/Teams/Internal/TeamChatMessageAttachment.cs	85.71% <0.00%> (-14.29%)	⬇️
...rc/sdk/PnP.Core/Services/Core/Query/QueryClient.cs	77.97% <0.00%> (-7.88%)	⬇️
....Core/QueryModel/Query/DataModelQueryTranslator.cs	77.61% <0.00%> (-7.11%)	⬇️
...sdk/PnP.Core/Model/Base/BaseDataModelCollection.cs	56.85% <0.00%> (-7.11%)	⬇️
.../PnP.Core/QueryModel/Query/ExpressionExtensions.cs	94.44% <0.00%> (-5.56%)	⬇️
...nP.Core/Services/Core/Http/MicrosoftGraphClient.cs	75.00% <0.00%> (-5.00%)	⬇️
... and 158 more

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