Implement correct priority of "-" in format strings under 1.9

Ruby 1.9 and 2.0 give higher priority to the "-" (left justify) operator
than to the "0" (pad with zeros) operator when used in format string in
conjunction with numeric values.

See http://ujihisa.blogspot.com/2009/12/string-differs-between-ruby-18-and-19.html
for details.
Merge pull request #2255 from leocassarani/string-modulo

Implement correct priority of "-" in format strings under 1.9