Product#meta_description should not fall back to a different locale

[mvz]

Plain Spree uses description as a fallback for meta_description to provide one of the meta-data tags in the header. As such, in the case of a missing translation when using spree_i18n, it makes more sense to show the description field for the same locale than fall back to a different locale.

Therefore, for this particular field, I think there should not be a fallback to a different locale.

[alepore]

Yeah, make sense, feel free to open a PR for this (on spree_globalize)