Fix string concatenation with a number as per rubyspec behaviour.

Following https://github.com/tychobrailleur/jcodings/commit/8536b4fdeecf3dec31ca868803654b3ab7829915, the incorrect behaviour of String#concat with
a number was made obvious by throwing an EncodingException.  MRI changes
the encoding of the String if the concatenated codepoint is within
ASCII-8BIT range, and throws a RangeError otherwise.