Make Content-Type json validation less rigid so it works with things like application/manifest+json (Fixes #983)

[felixsanz]

What is it?

• Feature / enhancement
• Bug
• Docs / tests

Description

There are Content-Types that are variations of application/json, like application/manifest+json, which is needed to output a route like /manifest.webmanifest.ts. This PR make it less rigid so it works with those variations.

Use cases and why

• 1. One use case

Using routes/manifest.webmanifest.ts needs to print application/manifest+json.

• 2. Another use case

application/ld+json and other variations.

Checklist:

• My code follows the developer guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• Added new tests to cover the fix / functionality

[adamdbradley]

great idea, thanks!