trailing spaces on Dates in dataframe

[peller]

I'm encountering dates in urth-core-dataframe that look like this:

"2013-02-01 15:13:00.000 "

Note the space at the last position. This succeeds in Chrome, but fails in Firefox and Safari.

For an example, look at fares in this notebook

[peller]

@poplav

[poplav]

Fixed by #512