Fix serialization of optional blobs.
Fix serialization of optional blobs.

Fixes #658

DualSpark/rusoto,mthjones/rusoto,mattico/rusoto,DualSpark/rust-aws,DualSpark/rust-aws,mattico/rusoto,mattico/rusoto,SecurityInsanity/rusoto,DualSpark/rusoto,DualSpark/rusoto,rusoto/rusoto,SecurityInsanity/rusoto,rusoto/rusoto,rusoto/rusoto,DualSpark/rust-aws,mthjones/rusoto