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 (closed)
• https://github.com/gitlabhq/gitlabhq/issues/7023

/cc @jacobvosmaer