Passthrough to_path for bodies when running specs in LINT mode

This allows running the sinatra_helpers send_file to_path spec.

However, as as Rack::Lint in older versions of Rack does
not passthrough body.to_path, only run the spec in Lint mode
on Rack 3+.

spec/plugin/sinatra_helpers_spec.rb

@@ -392,7 +392,7 @@ def o.each; yield 'Hello World' end
 
     it "returns response body implementing to_path" do
       req[2].to_path.must_equal @file
-    end unless ENV['LINT']
+    end if !ENV['LINT'] || Rack.release >= '3'
 
     it 'sets the Content-Type response header if a mime-type can be located' do
       header(RodaResponseHeaders::CONTENT_TYPE).must_equal 'text/css'

spec/spec_helper.rb

@@ -197,6 +197,7 @@ def _req(app, env)
       orig = a[2]
       a[2] = a[2].to_enum(:each).to_a
       orig.close if orig.respond_to?(:close)
+      a[2].define_singleton_method(:to_path){orig.to_path} if orig.respond_to?(:to_path)
     end
 
     a