Disable brace expansion inside heredocs (#147)

/bin/bash does not perform brace expansion on heredoc content:

  $ /bin/bash -c 'cat <<EOF
  {a,b}
  EOF'
  {a,b}

Without this fix, just-bash produces "a b" — the brace expansion
result. This corrupts any heredoc containing JSON or similar
brace-and-comma content.