[ticket/12133] fix download file names for WebKit-derived browsers
All browser engines that descend from WebKit declare themselves as
Safari in the user agent, including Chrome. Currently, the code
assumes that any Safari-like browser cannot handle the RFC-compliant
filename syntax. At least for recent versions of Safari, this is no
longer the case, and the legacy syntax results in URI-quoted
filenames. Using the standard syntax works as expected in both Safari
9 and Chrome 45.
The ticket reporting this issue is from January 2014, so we can safely
ignore compatibility: any browser still relying on the previous
behaviour is unlikely to receive security updates.
