fix code example

Code example references a direct call to `Error` which fails, proper example should us the `binding.Error` instead.
return `Content-Type: application/json` in `ErrorHandler

The `ErrorHandler` defaults to returning `text/plain` despite the fact that it actually returns JSON. I've set the `Content-Type` header to `application/json; charset=utf-8`.
Merge pull request #52 from danawoodman/patch-2

fix code example
Merge pull request #53 from danawoodman/patch-3

return `Content-Type: application/json` in `ErrorHandler
Error page handler