Add specs for String#upto on sequence of numbers
Fix String#upto on sequence of numbers for X19

MRI(ruby 1.9.3p392 (2013-02-22 revision 39386))
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    '9'.upto('10').to_a #=> ["9", "10"]

Rubinius(X19)
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    '9'.upto('10').to_a #=> []
Merge pull request #2309 from kachick/fix-string-upto-x19

String#upto with sequence of numbers should behave as Integer#upto for x19