Set buffer mode according to `auto-mode-alist'

With this, after we create a special buffer, the major-mode get set
automatically. For example, we create a new buffer named "ruby.rb",
`ruby-mode` was automatically enabled.
Respect initial-major-mode creating scratch buffer
Merge pull request #592 from toctan/set-buffer-major-mode

Set new special buffer mode according to `auto-mode-alist'