Handle virtual paths in FilePathResult

[javiercn]

The current implementation doesn't handle "~/" paths. This requires something like Server.MapPath
See #1190 and #429

[danroth27]

@loudej @davidfowl We need some way to figure out what file system we get.

[javiercn]

This should be provided to the action result via a constructor or resolved within the action result

[javiercn]

@davidfowl @Tratcher We need something like Server.MapPath exposed by the host, and this is probably also required for Razor, @pranavkm am I right?

[Tratcher]

@loudej? We were discussing this Friday.

[lodejard]

Yes, we should cover that on Wednesday too