Allow for optionally serving pre-compressed files.

[hcldan]

There wasn't a good way to propagate the new option into NamedFile so I made a new constructor and added a flag to the struct. In order to make the flag understandable, I converted to a struct with members instead of a simple tuple.

I am not sure how to test the Content-Type returned... It seems that the original tests didn't check those headers and there's not really a clean way to pass that in the assert functions without refactoring a bunch of stuff.