Issue #393 Adding missing fire event for onClose with file rendering

Issue #393 - Adding more tests and refactoring duplicated file transmitter code
Good question, we're using the given headers in master: https://github.com/ratpack/ratpack/blob/9907fb95cbde16be5fd8c8b6377881e0b6291210/ratpack-core/src/main/java/ratpack/file/internal/DefaultFileHttpTransmitter.java#L114.

I've updated here: https://github.com/danhyun/ratpack/commit/ef3dc5ac0d0aeb3c781c1aed1dec818e3b98624e