Fix generated MIME methods to recognize kwargs
Without this fix, generated MIME methods can't recognize kwargs:
```
% bin/test -w test/abstract/collector_test.rb:53
Running 5 tests in a single process (parallelization threshold is 50)
Run options: --seed 56716
# Running:
F
Failure:
AbstractController::Testing::TestCollector#test_generated_methods_call_custom_with_arguments_received [/Users/kamipo/src/github.com/rails/rails/actionpack/test/abstract/collector_test.rb:59]:
--- expected
+++ actual
@@ -1 +1 @@
-[#<Mime::Type:0xXXXXXX @synonyms=[], @symbol=:text, @string="text/plain", @hash=-3828881190283196326>, [:foo], {:bar=>:baz}, nil]
+[#<Mime::Type:0xXXXXXX @synonyms=[], @symbol=:text, @string="text/plain", @hash=-3828881190283196326>, [:foo, {:bar=>:baz}], {}, nil]
```
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