RFR: 8235459: HttpRequest.BodyPublishers#ofFile(Path) assumes the default file system

[Julia Boes]

Thanks for the review, Chris.

After running further httpclient tests, I had to adjusted the exception 
handling of an existing test, RelayingPublishers.

I also added a check in FilePublisher::createInputStream to throw if the 
path is of a directory.

Updated webrev: http://cr.openjdk.java.net/~jboes/webrevs/8235459/webrev.02/

Thanks,

Julia