Fix test suite for Rack 2.x compatiblity.

[voxik]

The test suite does not pass when testing against Rack 2.x. It fails with following error:

  1) Rack::Test::Utils build_multipart builds multipart bodies
     Failure/Error: params = Rack::Utils::Multipart.parse_multipart(env)
     NameError:
       uninitialized constant Rack::Utils::Multipart
     # ./spec/rack/test/utils_spec.rb:64:in `block (3 levels) in <top (required)>'

This is caused by this change rack/rack@b2d7396#diff-7c1a24d5b2fe58a6f925c7cacc6c55e7L573 in Rack.

[voxik]

@scepticulous you might want to incorporate this into your PR

[junaruga]

@voxik thanks for your pull-request. your commit was merged to master branch at #151 .