Fix spec covering the encoding of Array#inspect with 1.9

Array#inspect uses the encoding of the string returned by Object#inspect
ran against the first entry of the array. Object#inspect will call
String#inspect, which does _not_ take into consideration the encoding of
the String object but will rather rely on the default external encoding.

Hence the spec must be rewritten to take into consideration the default
external encoding.
Merge pull request #147 from flavio/fix_ascii_inspect_ruby_19

Fix spec covering the encoding of Array#inspect with 1.9