Fix AR::Base.quote_bound_value for AS::M::Chars values

Because of an explicit check on the String class Chars objects got mapped and quoted instead of just quoted.

So quote_bound_value("A\nB".chars) got converted to "'A','B'".

This patch adds String#acts_like_string? and AS::M::Chars#acts_like_string? to solve this in a maintainable manner.

Related to: #980