Lowercase content-type and x-cascade headers

[schinery]

Questions:

• The files spec/grape/entity_spec.rb and spec/grape/middleware/formatter_spec.rb has specs testing for Content-type, should these be lower case?

[grape-bot]

	1 Warning
⚠️	Unless you're refactoring existing code or improving documentation, please update CHANGELOG.md.

Here's an example of a CHANGELOG.md entry:

* [#2356](https://github.com/ruby-grape/grape/pull/2356): Lowercase content-type and x-cascade headers - [@schinery](https://github.com/schinery).

Generated by 🚫 Danger

[dblock]

@schinery There's likely a lot of people relying on the fact that these headers are capitalized the way they are in Grape. I am 100% sure I've written code like that. In the original issue you say "The X-Cascade header in Rack 3 applications appears to need to be lower case..." - can you please help find the source of that and why it was changed? If it's an RFC of some kind then we can convince ourselves it's the right change.

[schinery]

Closing as #2355 seems to be the preferred approach