many of the (even cheap) chargers wont charge backwards, and the Protection on the cell itself will also assist in such matters.
also the right charger design, does not allow reverse connection, refered to as mechanical polarity protection, where one of the connetion points does not reach connection to the battery when inserted improperly. this is NOT usually the case with the cheap li-ion chargers though, because of the flat heads (no nipple) batteries that can be charged with them.
The voltage was maintained, as you have checked , this all being good, you checking, and it being good still.
so in short, your good to go, but of course try and avoid that.