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

Issue #393 - Adding more tests and refactoring duplicated file transmitter code
Cool, wasn't sure how to use but I think I got it https://github.com/danhyun/ratpack/commit/b2e97ac0e0d9ca14c875a73ed41c1607ca1a1dff

Btw does `counter++; set(counter)` seem more clear than `set(++counter)`?