Adding StringComparison.InvariantCulture to IndexOf

[storno77]

Fixing issue #420 - adding StringComparison.InvariantCulture to IndexOf call, extending unittests to test this situation.

[jamesmh]

Thanks for this! I've got a few changes to suggest here:

1. First, there are other places within the cron expression code that will require the same fix. Notably, in the CronExpressionComplexPart class too.

2. Because of this, I think the more appropriate approach to testing here is the following:

   1. Do the tests from SchedulerCronTests.cs instead so we can cover a broader range of cron expressions and internal logic to verify.
   2. Copy the existing 1 test (including all the InlineData attributes) and create a new test where you can assign a different culture to each of the cron expressions being tested and within the test code adjust the current culture - just like you did in your PR before these changes.

Let me know if that makes sense or if you need any help 👍

[storno77]

Hi, I've pushed the changes you wanted, please check.