Force encoding of posix_spawn output

Currently the encoding of the output of `POSIX::Spawn::Child.new(*command, :input => target)` in https://github.com/github/markup/blob/master/lib/github/markup/command_implementation.rb#L36 is always `ASCII-8Bit although the input encoding might be different (e.g. `UTF-8`).

This PR forces the encoding of the output of the call to be the same as the input data.

/cc @jacobvosmaer