GH-1043 Add support for communicating type via MimeType parameter for JsonMessageConverter

Resolves #1043

docs/src/main/asciidoc/spring-cloud-function.adoc

@@ -608,7 +608,7 @@ up to the actual implementation of the `MessageConverter` to support multiple ty
 As mentioned earlier, the framework already provides a stack of `MessageConverters` to handle most common use cases.
 The following list describes the provided `MessageConverters`, in order of precedence (the first `MessageConverter` that works is used):
 
-. `JsonMessageConverter`:  Supports conversion of the payload of the `Message` to/from POJO for cases when `contentType` is `application/json` using Jackson or Gson libraries (DEFAULT).
+. `JsonMessageConverter`:  Supports conversion of the payload of the `Message` to/from POJO for cases when `contentType` is `application/json` using Jackson (DEFAULT) or Gson libraries. This message converter also aware of `type` parameter (e.g., _application/json;type=foo.bar.Person_). This is useful for cases where types may noyt be known at the time when function is developed hecn function signature may look like `Function<?, ?>` or `Function` or `Function<Object, Object>`. In other words for type conversion we typically derive type from function signature. Having, mime-type parameter allows you to communicate type in a more dynamic way.  
 . `ByteArrayMessageConverter`: Supports conversion of the payload of the `Message` from `byte[]` to `byte[]` for cases when `contentType` is `application/octet-stream`. It is essentially a pass through and exists primarily for backward compatibility.
 . `StringMessageConverter`: Supports conversion of any type to a `String` when `contentType` is `text/plain`.