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* GH-3280: NullChannel as reply for void gateways Fixes https://github.com/spring-projects/spring-integration/issues/3280 To properly support a one-way gateway (`void` return type), it is better to ignore any possible replies from downstream instead of unexpected `no output-channel or replyChannel header available` error * Populate a `nullChannel` into a `replyChannel` header for `void` gateway methods when `replyChannel` is not set explicitly * Remove redundant `GatewayProxyFactoryBean.PARSER` in favor of similar `EXPRESSION_PARSER` in the super class * Test and document the feature * * Fix language in the `whats-new.adoc` Co-authored-by: Gary Russell <grussell@vmware.com> * * Fix language in gateway.adoc Co-authored-by: Gary Russell <grussell@vmware.com> Co-authored-by: Gary Russell <grussell@vmware.com>