Introduce Hanami::Middleware::BodyParser::Parser

[jodosha]

What?

Introduce Hanami::Middleware::BodyParser::Parser as superclass for body parsers

Why?

By reviewing this documentation fix, I've recognized that in unstable branch the superclass for body parsers is gone.

Since 1.3, we put in place a mechanism to check if a given body parser respond to the required methods (see #182). In a language with missing interfaces, this is the best that we can do.

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However, we cannot easily document the behavior of these methods.

For instance, we currently assert that a parser responds to #mime_types, but we can't document that we expect an array of strings as returning value.

Same thing goes for #parse, which is even more complex due to the expected exception to be raised.

How?

The superclass is used by our only body parser for JSON.

It doesn't change the current possibility of having a body parser that implicitly inherits from Object. This PR only indicates the preferred way.