reduce AS::SafeBuffer allocations

html_escape_interpolated_argument is only used in mutation methods:

  https://github.com/rails/rails/blob/c07d09559ec171e1904b55c7ad7e8c7d586ca51b/activesupport/lib/active_support/core_ext/string/output_safety.rb#L174
  https://github.com/rails/rails/blob/c07d09559ec171e1904b55c7ad7e8c7d586ca51b/activesupport/lib/active_support/core_ext/string/output_safety.rb#L179

The return value doesn't need to be converted to an AS::SafeBuffer since
we know that the current object is an AS::SafeBuffer and will be
mutated, and the return value from html_escape_interpolated_argument
will be thrown away