This is needed to address a few outstanding CVEs and fix cookie
timestamp formats.

Full list of changes:
https://github.com/rack/rack/compare/2.0.9..2.1.4

Relates to:
* #36362
* #228622

Rack v2.1.x no longer coerces the body to a string. The Rack spec
(https://github.com/rack/rack/blob/master/SPEC.rdoc#the-body-) says:

The Body must respond to `each` and must only yield String values

Previously in a few places the Grape API was returning the status code
as an integer, which Grape used as the response body. To preserve the
legacy behavior, we explicitly set the body to the stringified integer.

In #267598, we saw Maven
packages report 500 errors because a `nil` body was being returned. This
has been fixed in
!45271.

Because it's easy to mistakenly return the wrong type in the Grape body,
this commit also adds a new Grape middleware that will automatically
coerce values to strings but raise an exception in development and test.

This reverts !45246
and brings back
!44518.