Fix excess precision of quantity in csv export of investment accounts.

Last August, for bug 473082, I made changes to increase the precision of some of the values written by the CSV exporter. Unfortunately, there remained one place where I called CsvWriter::format with the smallestAccountFraction instead of the actual precision, resulting in quantities with 10000 decimal digits being output (all 0s). This changes "strQuantity = format(qty, security.smallestAccountFraction());" to "strQuantity = format(qty, MyMoneyMoney::denomToPrec(acc.fraction()));" in two places in csvwriter.cpp. This change had actually been suggested by Thomas in an email in August, and I have no idea why I didn't use it then. (The changes in August were to both 5.1 and master branches. I'm only submitting to master branch for now, unless we get any reports against 5.1)