Fix encoding of output of markup, prevent 500 for non-ascii characters

#### What does this MR do?

This MR forces the output of `Github::markup` to have the same encoding as the input to it.

#### Why was this MR needed?

This prevents 500 errors if the markup contains non-asci characters.

#### What are the relevant issue numbers / Feature requests?

This MR closes

* #296
* https://github.com/gitlabhq/gitlabhq/issues/7023

/cc @jacobvosmaer

See merge request !160