Fix string type errors function

[artemnvkv]

Added an additional check is_binary before String.valid?, because since version 15 of Elixir, the function String.valid? only accepts strings, otherwise we get an error

    ** (FunctionClauseError) no function clause matching in String.valid?/2

     The following arguments were given to String.valid?/2:

         # 1
         1.0

         # 2
         :default

     Attempted function clauses (showing 2 out of 2):

         def valid?(<<string::binary>>, :default)
         def valid?(<<string::binary>>, :fast_ascii)