Avoid overwriting Hash#deep_merge

Monkey-patching Hash to define deep_merge there without checking might be danger because it would conflict with ActiveSupport version of `Hash#deep_merge` https://github.com/rails/rails/blob/94b7328b08fcc55e82bfcaa34a25ae718921ae1c/activesupport/lib/active_support/core_ext/hash/deep_merge.rb#L16