Formatter responsibility should only be to format the error, not to handle the request

[ilariopro]

Hi, I think this library is great, but why the formatters are responsible to return the final response?

I don't understand this design choice, mainly for these reasons:

1. I consider this a violation of SRP: the formatter should only be responsible for formatting the error and returning it to the ErrorHandler. It should not depend on the ResponseFactory and StreamFactory.
2. Currently, each FormatterInterface implementation depends on ResponseFactory and StreamFactory. This is acceptable when using the Factory helper, but it makes the dev experience difficult if you need to specify custom implementations.
3. Potentially you can specify a different ResponseFactory/StreamFactory implementation for each formatter.

I could prepare a PR for this change, if you want. But it would be potentially a breaking change.

[oscarotero]

The original idea was to provide full flexibility, even using different factories per formatter (why not?).
But I see your point. All formatters extend AbstractFormatter but maybe that code shouldn't belong to the FormatterInterface in the first place.

What do you think @filisko ?