Regress after unparsed case expressions

Resolves https://github.com/playframework/twirl/issues/26

This aligns the behaviour with the old parser, which looks for a complete parse of `case (.+)=>` and a block.

A similar change was already made for match expressions: https://github.com/playframework/twirl/commit/b160c59a2dafbfd5a76e872ce3f44c68fb3f342f