* ext/objspace/objspace_dump.c (dump_append_string_value): Escape
  control characters for strict json parsers.
* ext/objspace/objspace_dump.c (objspace_dump): Document File/IO
  output option.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43852 b2dd03c8-39d4-4d8f-98ff-823fe69b080e