Allow access to unsanitized file name

[j0hax]

Hello there! I've come across a situation where I have to save a file with a "double" extension (*.kepub.epub), and the current implementation of (r *Response) FileName() purposefully breaks that by always calling SanitizeFileName().

I propose to provide direct access to an unsanitized file name with the method RawFileName(). This shouldn't affect compatibility and allows for accessing information similar to URL package's RawPath, RawQuery, etc.

[WGH-]

Ehm, perhaps, sanitizing double extension away was a mistake in the first place.