Micro optimization: Declare closures in BodyParsingMiddleware as static

[Ayesh]

All three anonymous functions used in BodyParsingMiddleware::registerDefaultBodyParsers() method never uses $this, so we can declare them static to apply a micro-optimization, which makes engine not bind the $this object to the closure.

[coveralls]

Coverage remained the same at 100.0% when pulling cfc6f10 on Ayesh:static-closures-pr into b081ebd on slimphp:4.x.