OptionParser show_defaults rounding error

[terencewtli]

When printing the default values from OptionParser, double values are formatted with extra zeroes. For example, setting a default value to 1.0e10 leads to OptionParser printing 10000000000.000000 instead. There's also an issue with rounding that causes a 'round-tripping' problem - default value 1234500000000.1200000034 gets printed as 1234500000000.120117.