Date.parse should not accept second values of "60"

[Jordan Harband]

`Date.parse('2012-12-31T23:59:60.000Z')` should return NaN, and instead returns `Date.parse('2013-01-01T00:00:00.000Z')`
`Date.parse('2015-06-30T23:59:60.000Z')` should return NaN, and instead returns `Date.parse('2015-07-01T00:00:00.000Z')`

Note that the second example is an actual leap second, and it doesn't even work properly.

The spec requires that only seconds from 0 to 59 are supported.

[Jordan Harband]

In latest Safari, both of these return NaN, so it must have been fixed at some point.