PIM-2494 : update the product value normalizer to remove null collection
from Mongo Normalized data
PIM-2494 : fix the price normalizer
PIM-2494 : add metric normalizer and related spec
PIM-2494 : update change log
PIM-2494 : fix product value normalizer spec
PIM-2494 : remove useless method
Merge pull request #1056 from akeneo/PIM-2494

PIM-2494 : update normalizers to remove null values from MongoDB normalized data