Detect null time-to-reference when calculating zero rates

[lballabio]

We need to detect and manage the case in which a date is not the same as the reference date of the curve, but the time between them is null according to the chosen day-count convention.

Fixes #1797.

[coveralls]

coverage: 71.913% (+0.001%) from 71.912% when pulling 6dc78b2 on zero-rate-check into 75accb5 on master.