computeCount() incorrect for headers that include embedded newlines

In computeCount() for MigrateSourceCSV there is an issue in the code for embedded newlines.

The first is that the comment for skipping headers is misleading ("Skip all but the last header": it isn't, it's skipping all of them). The second is that the loop to skip headers is using fgets(), which doesn't take into account embedded newlines itself, so if there are any in the header row it will skip the wrong number. The code should be using $this->getNextLine() instead.