INT-3142 Allow Merging of HTTP Message Converters

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

* Introduced a new flag in the 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

Changes per feedback from Artem - additional javadoc comments, cleaned up tests

Updated: Setting the register-default-converters by default to false now, for backward compatibility reasons
Changed adapter name per feedback from Artem

Changed adapter name per feedback from Artem

Changed attribute name from register-default-converters to merge-with-default-converters

INT-3142: Atomic operation for messageConverters

'What's New' polishing

Polishing

Fix a few comments, error messages, docs, whitespace.