Merge pull request #2017 from suzukaze/fix_enumerator.inspect

Add args in Enumerator#inspect
wrap args by parens
should have passed args to to_enum
Delete khash_mask macro.
And fix expression to more simple and readable.
Use kh_end instead to n_buckets
Merge pull request #2022 from ksss/khash-delete-khash_mask

Delete khash_mask macro
Merge pull request #2023 from ksss/khash-readable

Use kh_end instead to n_buckets
Hash#replace should copy default from original even when the default value of the original is not set
Merge branch 'master' of github.com:mruby/mruby
Fix C++ files detection for automatic C++ ABI enabling.
Merge pull request #2025 from take-cheeze/cxx_abi_fix

Fix C++ files detection for automatic C++ ABI enabling.
Add MRuby::Build#disable_cxx_exception to force C++ ABI disable.
Add documentation of MRuby::Build#disable_cxx_exception .
Merge pull request #2026 from take-cheeze/disable_cxx_exception

Add MRuby::Build#disable_cxx_exception to force C++ ABI disable.
Fix wrong `mrb_bug` use found with mruby-clang-plugin.
Fix build_config.rb for ArduinoDue
Merge pull request #2030 from kyab/fix-build_config-arduinodue

Fix build_config.rb for ArduinoDue
Merge pull request #2029 from take-cheeze/mrb_bug_fix

Fix wrong `mrb_bug` use found with mruby-clang-plugin.
mention 1.9 compatibility in README.md; close #2031
Merge branch 'master' of github.com:mruby/mruby