force mode of uploaded profile banners to be 0644 (graphicsmagick seems to ignore umask)
FileUtils calls expect a string not a Media::Tempfile

output_file is not guaranteed to be a String. So let's call to_s on it. Also makes the asset test pass.
Merge pull request #72 from Crabgrass/bugfix/profile-banners

force mode of uploaded profile banners to be 0644