Prior to this commit, FreeMarkerView used the system default charset to
render. This commit switches this by defaulting to UTF-8, if no charset
is specified in the content type.

 - Add contentType parameter to AbstractView.renderInternal, used to
 determine the charset contained therein
 - Adds a defaultCharset property to AbstractView and
 ViewResolverSupport.

Removed in favor of accessing the factory from the response.

While View and ViewResolver play the same role as in spring-webmvc they
are now abstracted behind the HandlerResultHandler abstraction so that
top-level contracts don't reference them and the DispatcherHandler is
also unaware of their existence.

Furthermore view resolution and response body handling which are now at
the same level of abstraction (each is a HandlerResultHandler) will
also share code for content negotiation, so it makes sense for them to
be side by side.

This commit moves the reactive.view package to reactive.result.view
with the View and ViewResolver contracts (previously in the top-level
reactive package) also moving there.