Fixed content-type serialization for lists

[timocov]

Issue #, if available:

Description of changes:

I guess it is a follow-up after #1043.

Considering you have the following model:

operation Op {
  output := {
    @httpPayload
    content: StringList
  }
}

list StringList {
  member: String
}

When you run a serialization by using code similar to

smithy-java/aws/server/aws-server-restjson/src/main/java/software/amazon/smithy/java/aws/server/restjson/AwsRestJson1Protocol.java

Lines 163 to 167 in c965198

	ResponseSerializer serializer = httpBinding.responseSerializer()
	.operation(job.operation().getApiOperation())
	.payloadCodec(codec)
	.payloadMediaType("application/json")
	.shapeValue(output);

it gets serialized with content-type header set to text/plain, while it should be the payload content type from the serializer. This fixes this issue.

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