Add Body Parsing Middleware reference

[cvigorsICBF]

Mention the requirement for Body Parsing Middleware when parsing application/json requests

Q	A
Documentation	yes
Bugfix	no
BC Break	no
New Feature	no
RFC	no
QA	no

Description

When requesting an access token from the authentication server using JSON, the response from the server will always be "error": "unsupported_grant_type" unless the body parsing middleware helper is included in the application. Updating the docs to highlight the requirement for this library.

Fixes #39

[cvigorsICBF]

Header added as requested

[cvigorsICBF]

Example provided as requested

[weierophinney]

Generally speaking, we recommend AGAINST piping the body parsing middleware as generic middleware; instead, we recommend piping it into a route-specific pipeline:

use Mezzio\Authentication\OAuth2;
use Mezzio\Helper\BodyParams\BodyParamsMiddleware;

$app->post('/oauth2/token', [
    BodyParamsMiddleware::class,
    OAuth2\TokenEndpointHandler::class
], 'auth.token');

This ensures we only parse it when we are actually expecting a content body.

[weierophinney]

This should be reverted; it adds an extra line between content, which our own linter will reject.

[froschdesign]

The correct way of usage must be shown, not the wrong one.

Suggested change

	For example:
	```php
	use Mezzio\Helper\BodyParams\BodyParamsMiddleware;
	$app->pipe(BodyParamsMiddleware::class);
	```
	Generally speaking, we recommend AGAINST piping the body parsing middleware as generic middleware; instead, we recommend piping it into a route-specific pipeline:
	```php
	use Mezzio\Authentication\OAuth2;
	use Mezzio\Helper\BodyParams\BodyParamsMiddleware;
	$app->post('/oauth2/token', [
	BodyParamsMiddleware::class,
	OAuth2\TokenEndpointHandler::class
	], 'auth.token');
	```
	This ensures we only parse it when we are actually expecting a content body.
	For example:
	```php
	use Mezzio\Authentication\OAuth2;
	use Mezzio\Helper\BodyParams\BodyParamsMiddleware;
	$app->post('/oauth2/token', [
	BodyParamsMiddleware::class,
	OAuth2\TokenEndpointHandler::class
	], 'auth.token');
	```
	WARNING: Do not piping the body parsing middleware as generic middleware.
	This ensures that the content body is only parsed when it is actually expected.