Fix Content and Product with no arch.

If a custom Product had no arch attribute, Content
with no arch attribute were being filtered. Flip
the filter logic so it defaults to adding content
and doesn't add it for arch mismatch. Content
with no arch attributes (directly or inherited
from Product) are not filtered out.
Merge pull request #282 from candlepin/alikins/arch_content_product_no_arch

Fix Content and Product with no arch.