fix possible arena overflow
add tests for String#capitalize! and String#downcase!
add test(requires MRB_GC_FIXED_ARENA enabled)
fix the position of capitalize! and downcast!, and add test for upcase!
Merge pull request #1723 from akuroda/string_test_cap_down

add tests for String#capitalize!, String#downcase!, and String#upcase!
Merge pull request #1722 from take-cheeze/hash_ext_arena_fix

Fix possible arena overflow in mruby-hast-ext.