fileupload dependency bump, with FileItemHeaders-support

Mostly to shut sbt up about conflicting dependencies in a project I'm working on. 
I took the liberty of removing two casts which i couldn't understand the purpose of as well.

(I think all the tests still run green, on os x they don't shutdown cleanly)
Using a var means keeping the object passed to us in setHeaders(), as opposed to initializing a FileHeadersImpl of our own and call (the not specified in interface, synchronous) addHeader() on it. 

As far as i could tell, this header mechanism must have been added in this way to keep compatibility, and is called directly after object creation. I instantiate a default one to avoid ever returning null, though i dont know if its needed. 

Should i go the val/mutate object route?