Test non-UTF-8-encoded files that contain EF BB BF

Byte sequences that look like a UTF-8-encoded BOM must only be stripped from UTF-8-encoded files. This patch makes sure we actually test if stripping non-UTF-8 files is successfully prevented.
Merge pull request #1 from mathiasbynens/fix-tests

Test non-UTF-8-encoded files that contain EF BB BF