integration: Fix test failures

integration-tests/req-set-header.rb

-Nghttpx.run do |resp, req|
-  req.set_header "User-Agent", "mruby"
+Nghttpx.run do |env|
+  env.req.set_header "User-Agent", "mruby"
 end

integration-tests/resp-set-header.rb

-Nghttpx.run do |resp, req|
-  resp.set_header "Alpha", "bravo"
+Nghttpx.run do |env|
+  env.resp.set_header "Alpha", "bravo"
 end

integration-tests/return.rb

-Nghttpx.run do |resp, req|
+Nghttpx.run do |env|
+  resp = env.resp
+
   resp.clear_headers
   resp.status = 404
   resp.add_header "from", "mruby"