Parse browser form data in schema controllers

[crimson-knight]

Summary

• route schema request parsing through Amber's registered content-type parsers
• parse the body before route matching can consume browser form data while checking _method
• cover merged URL-encoded browser form fields alongside query data

This fixes the framework side of Amber V2's database-backed default web scaffold. The generated HTML form uses application/x-www-form-urlencoded; before this change it rendered correctly but its submitted fields never reached the schema.

Validation

• controller/schema form integration: 13 examples, 0 failures
• Crystal formatting: changed source and spec clean