Fix incorrect expiration of value if expiration_time is -1.

According to the documentation below, setting expiration_time to -1 should get a cached value with no expiration consideration. However, the methods did not seem to behave this way.

http://dogpilecache.readthedocs.org/en/latest/api.html#dogpile.cache.region.CacheRegion.get_or_create.params.expiration_time http://dogpilecache.readthedocs.org/en/latest/api.html#dogpile.cache.region.CacheRegion.get_or_create_multi.params.expiration_time