allow seconds as Rational in Time.local and set nanoseconds from it
fix logic of RubyDateFormat for %N, it is right zero-padded (String.format() cannot do that)
use twoCharDigit() helper instead of inlined logic
Trade tabs for spaces in date lib *.rb files

* MRI did remove tabs in *.rb but after date was made native
* Much easier to edit
* Merge with MRI will not happen anymore anyway
Fix Time#round

* account for sub-ms value
* maybe refactor the first line in a helper?
* use integer division, no conversion to double and back
Time#round is supposed to return a Time instance
Really fix Time#round: handle overflow and use plusMillis() instead of withMillisOfSecond() directly