Restore overlay

[mvz]

This pull requests does a couple of things:

• Restore mime-type specific tests that were commented out
• Restore the file lib/mimemagic/overlay.rb to its original version from the 0.3.x series. This does two things:
  • Make the tests for Office 2007+ file types pass
  • Make dependent gems that use this file not fail to load (e.g., caxlsx)
• Update tests for recognizing *.html files; due to a change in the mime type database these are now recognized as application/xhtml+xml.