Fix behavior of trying to parse non-string objects

[YuheiNakasaka]

Some Object may return true for respond_to?(:to_str) but actually return nil instead of String when to_str is called.

A good example is the Rails encrypted credentials in Rails 7.0, an object called InheritableOptions that extends Hash.
InheritableOptions implements method_missing and returns true for respond_to?(:to_str) but nil when actually calling to_str.
https://github.com/rails/rails/blob/7-0-stable/activesupport/lib/active_support/ordered_options.rb#L43

Therefore, the following implementation will cause an error.

h = ActiveSupport::InheritableOptions.new({ a: 1, b: 2 })
h.is_a? Hash #=> true
JSON(h) #=> TypeError: no implicit conversion of nil into String

This PR implements strict checking of arguments to avoid this kind of error.