Fix Rack::Lint

[ericproulx]

Motivation

Rack::Lint is a middleware that validates applications, requests and responses according to Rack spec. I thought it would be great to have an idea of our compliance with the newest specs. Without any surprises, I've found a few violations but nothing major and most violations were found in specs.

This PR fixes all violations but do not prevent adding new ones. We would have to change a couple of things in the specs to add Rack::Lint in the specs but its another discussion.

Here what has been fixed:

Body yielded non-string value nil

# spec/grape/endpoint_spec.rb:848 
# spec/grape/endpoint_spec.rb:878

Under specific case, Grape might return a body equal to [nil] which is not valid according to the linter. Instead of relying on the received bodies, it returns systematically [] like Rack::Response does. See formatter

Body yielded non-string value :symbol, exception

# spec/grape/endpoint_spec.rb:811 
# spec/grape/endpoint_spec.rb:795
# spec/grape/api_spec.rb:1849 
# spec/grape/api_spec.rb:2081

When calling error! a user might not pass a string message. I've changed a bit the algorithm to make sure it's always a string. See txt.

Rewindable input

According to Rack 3 specs, input may not be rewindable. Instead of calling rewind all the time, I've added an if input.respond_to?(:rewind). See formatter

Body has not been closed

When cascading, we need to close the body if it can be closed respond_to?(:close)
See router