Fix `Encoding::CompatibilityError` of edit view.

hi.

`Encoding::CompatibilityError - incompatible character encodings: UTF-8 and ASCII-8BIT:` occurs in the edit view.

 - Add `_Header.md` `_Footer.md` `_Sidebar.md` that contains multibyte characters in the repository for Wiki
 - Run gollum by specifying `--no-live-preview`
 - Add page that contains multibyte characters in gollum
 - Open edit view on Edit button.
 - `Encoding::CompatibilityError`

I have fixed by changing to `page.text_data` from `page.raw_data` in `views/edit.rb`.