RestTestClient returns null response body

[rstoyanchev]

DefaultResponseSpec decodes the body via methods on ExchangeResult that in turn delegate to the client response, and then creates an EntityExchangeResult with the decoded body. However, the methods on ExchangeResult are public can be called again even though the body can only be consumed once.

ExchangeResult needs to be updated to not expose any public getBody methods, which should only ever be done through the subclass EntityExchangeResult.