Fix product import sku field data type bug

[jurgisl]

Fixes #7904. Always convert SKU to string.

Without the quotes SKU field on the left is being converted to numeric so it drops trailing characters. Then the numeric comparison sees the 13020663105 and 13020663105SL as the same numbers.

This happens because earlier SKUs are being used as array keys, and array keys, if it is possible, are converted to integer by PHP http://php.net/manual/en/language.types.array.php:

Strings containing valid integers will be cast to the integer type. E.g. the key "8" will actually be stored under 8. On the other hand "08" will not be cast, as it isn't a valid decimal integer.

So if the function is using these keys, they must be manually cast to string (because all SKUs should always be treated as string?).

[magento-cicd2]

All committers have signed the CLA.

[driskell]

I can see this conflicts as it is now fixed in develop:

c2c8abb

[maghamed]

Closed as fix for this issue already delivered into mainline

[tkn98]

Ack.