Fix application crashes if there are two environment variables with same name, differing only in case

[ron-myers]

Summary

This corrects the bug identified in #64.

Approach

The solution to this was less obvious than I expected when I sat down to build this.

As I saw it, there were a few options here:

• treat env keys as OrdinalIgnoreCase and pull the first value
• treat env keys as OrdinalIgnoreCase and pull the last value
• move to ordinal string comparison for env keys

I then read:

https://docs.microsoft.com/en-us/dotnet/api/system.environment.getenvironmentvariable

which includes:

To retrieve all environment variables along with their values, call the GetEnvironmentVariables method. 
Environment variable names are case-sensitive on Linux and macOS but are not case-sensitive on Windows.

Then attempted to set two environment variables on Windows that only differed by case and did not find a way to do so.

So decided to approach with Ordinal string comparison and submit for discussion.

Going with this approach required a change in CommandOptionsSchema

Added a unit test to verify the behaviour.

Happy to iterate if you have a preferred approach.

Fixes #64

[Tyrrrz]

Hi, thanks for PR!

I guess the name of test Environment_variables_pulled_ignoring_case is wrong, it should say "while_respecting_case", right?

Please also add a comment about case sensitivity on different OSes to CliApplication.RunAsync where the environment variables are extracted. It would be useful to know in the future why this decision was made.

[Tyrrrz]

Also, this part will need to be updated as well:

https://github.com/Tyrrrz/CliFx/blob/b4918187796a85a2a925f11c2292d6004ef2c1cc/CliFx.Analyzers/CommandSchemaAnalyzer.cs#L207-L212

[Tyrrrz]

Thanks!