Add missing `Content-Type` headers to `@httpPayload` protocol tests

[david-perez]

As per the specs for these protocols, a Content-Type header should be set. Servers should reject requests when an expected Content-Type header is not found (protocol tests for that are added in #2314).

• https://smithy.io/2.0/aws/protocols/aws-restjson1-protocol.html#content-type
• https://smithy.io/2.0/aws/protocols/aws-restxml-protocol.html#content-type

Testing

Ran tests in smithy-rs.

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