Improving jsonContentType Regex

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I was getting a PR ready for this, but it said to discuss via Issues first, so here I am. I'm pretty sure this is a good fix, but let me know if I am wrong.

Original:
jsonContentType: new RegExp('^application/[a-zA-Z.]+\\+json') (Link)
My proposed change:
jsonContentType: new RegExp('^application/[a-z0-9.]+\\+json$', 'i')

The original expression was incorrect because it was not checking 0-9, so a valid Content-Type like this would not validate: application/vnd.Microsoft.IIS.Administration.Files.2.2.0+json

Also added the ending $ for completion, and replaced A-Z with the i flag to account for mixed case letters, and also the possibility of mixed case throughout the entire Content-Type.

Are you willing and able to fix this?

Yes

Start with lib/plugins/utils/regex.js at the jsonContentType expression linked in the issue. Check the proposed pattern against the supplied mixed-case Content-Type example and existing validation behavior; done means valid application types ending in +json are accepted without accepting trailing content.