Fix ByteSize not handling default formatting correctly

[smoogipoo]

Fixes #953

• 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 ReSharper warnings
• There is proper unit test coverage
• 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

Turns out that Humanizer was handling formatting wrong in two ways, unlike the issue mentions.

1. The .NET spec requires the "G" format to be handled. I'm not sure if what I've chosen is the correct default, please advise if not.
2. Similarly the class inadequately handled the null provider. The same MSDN article defaults to the current culture in its samples, which I'd say is a sane default.

The added test will test both of these scenarios, by providing ("G", null) as args.

[dnfadmin]

All CLA requirements met.