INT-3142: A flag to retain default converters when user has customized message converters for HttpRequestHandlingController, HttpInboundGateway

INT-3142: A flag to retain default converters when user has customized message converters for HttpRequestHandlingController, HttpInboundGateway

* Introduced a new flag in base class of `HttpRequHttpRequestHandlingController`, `HttpInboundGateway` - `registerDefaultConverters`
* Introduced a new attribute `register-default-converters` for `inbound-channel-adapter` and `inbound-gateway` to indicate if the default `HttpMessageConverter`'s need to be registered
* Http namespace parser related changes to set the new flag
* Added additional tests to test the flag and the namespace parser related changes

Reference:
https://jira.springsource.org/browse/INT-3142