Fix htmlPath issue
Don't escape text/plain as HTML in default error handlers

Before: `Cannot GET http://localhost/?a=b&amp;c=d`
After:  `Cannot GET http://localhost/?a=b&c=d`
Merge pull request #875 from johan/html-404

Send 404 errors in a way all browsers decode right
Merge pull request #730 from JacksonTian/html_path

Fix htmlPath issue