Fixes #4905: Error while trying to set "weight" in Variant with "," as s...

...eperator.
Like this? https://github.com/aleks/spree/commit/19b0e5249fa0ce506a864d41d4734d06c9a41016

I'm not sure how to handle this for other variant values (like height, weight and depth), i don't think cloning parse_weight for each value is a good idea. :)