Adjust IndexOf Usage

[dahlbyk]

Nerd-sniped myself this morning:

1. Noticed a use of CurrentCultureIgnoreCase in the presence of an IFormatProvider. Since most IFormatProviders are CultureInfo, we can use their CompareInfo.
2. Noticed a few uses of str.IndexOf(...) == 0 which can be more efficiently written as str.StartsWith(...).

Checklist

Here is a checklist you should tick through before submitting a pull request:

• Implementation is clean
• Code adheres to the existing coding standards; e.g. no curlies for one-line blocks, no redundant empty lines between methods or code blocks, spaces rather than tabs, etc.
• No Code Analysis warnings
• There is proper unit test coverage
  • Not sure if it's worth writing a unit test for the CurrentCultureIgnoreCase change? Existing tests pass.
• If the code is copied from StackOverflow (or a blog or OSS) full disclosure is included. That includes required license files and/or file headers explaining where the code came from with proper attribution
• There are very few or no comments (because comments shouldn't be needed if you write clean code)
• Xml documentation is added/updated for the addition/change
• Your PR is (re)based on top of the latest commits from the dev branch (more info below)
• Link to the issue(s) you're fixing from your PR description. Use fixes #<the issue number>
• Readme is updated if you change an existing feature or add a new one
• Run either build.cmd or build.ps1 and ensure there are no test failures