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.