Add Content-Length header for static content

Nancy does not set Content-Length header for static content. Setting of Content-Length was introduced in in this commit: 3220e472128b78544e6e3c35ff07d0165a665273. However this change was lost in the next commit: 0a7a8f4872c4dffc6b09b4ff72896805afab264c. This pull request re-introduces setting of Content-Length in GenericFileResponse and adds a unit test in GenericFileResponseFixture.
I know it's out-of-scope for the change you have submitted, but could you perhaps help us out by sorting out the mixed use of tabs and spaces in this file? Would safe us from revisiting the file after accepting this pull-request.

Thanks!