Smithy 1.13 (#823)

* Bump smithy version to 1.13

* Upgrade to Smithy 1.13

The Smith 1.13 upgrade adds new restJson protocol tests. These demonstrated two issues:

1. We were not properly using the http binding index to compute content types. We should not have fallen back to a default content type—instead we need to use exactly what is given. We were also using the index incorrectly: `document` in the context of the index refers to anything in the HTTP body, _not_ the Smithy document type.
2. We were sending `Content-Length: 0` even when no HTTP body was modeled.
3. We were sending an empty body for structures targetted `@httpPayload` trait in restJson. We should instead be sending on an empty JSON body: `{}`.

* fixup indentation

* Fix eventstream content types

* Update S3 Signing test

ListObjectsV2 was manually verified to function correctly with the updated headers.