Fix specs for Rack Edge

[ericproulx]

We need to fix some tests for rack_edge

1) Grape::API format streams the content of the file with stream
     Failure/Error: subject.use Rack::Chunked

     NameError:
       uninitialized constant Rack::Chunked
     # ./spec/grape/api_spec.rb:1255:in `block (3 levels) in <top (required)>'

  2) Grape::Middleware::Base#response when Array header
     Failure/Error: expect(subject.response.header).to have_key(:abc)

     NoMethodError:
       undefined method `header' for an instance of Rack::Response
     # ./spec/grape/middleware/base_spec.rb:96:in `block (4 levels) in <top (required)>'

  3) Grape::Middleware::Base#response when Rack::Response header
     Failure/Error: expect(subject.response.header).to have_key(:abc)

     NoMethodError:
       undefined method `header' for an instance of Rack::Response
     # ./spec/grape/middleware/base_spec.rb:118:in `block (4 levels) in <top (required)>'