Consider `NaN` to be equal to itself in server protocol tests (#1177)

The client protocol tests use the
`aws_smithy_protocol_test::FloatEquals` for this [0].

Note we're only applying this to direct floating point shape members,
i.e. this commit _does not_ address #1147.

[0]: https://docs.rs/aws-smithy-protocol-test/latest/aws_smithy_protocol_test/trait.FloatEquals.html