BUG: Use dt as a max value in trading get_index.

Instead of sliding to the next trading day because of the behavior
of `searchsorted`, if dt argument is not a trading day use it as a
max value for corresponding date of the index.

Fixes a bug where if the end of the quarter is calculated with
disregard to trading days, get_index would return the first day
of the next quarter, instead of the last trading day of the intended
quarter.