go18n extract badly handels escapted characters

[hanzei]

If a message contains a escaped character e.g. " go18n extract escapes it again.

E.g.: If you have two messages:

	a = &i18n.Message{
		ID:    "a",
		Other: "a \" b",
	}
	b = &i18n.Message{
		ID:    "b",
		Other: `a " b`,
	}

go18n extract creates the following files:

json

  "a": "a \\\" b",
  "b": "a \" b",

toml

"a" = "a \\\" b"
"b" = "a \" b"

yaml

a: a \" b
b: a " b

As you can see the extracted messages for b are correct, but for a they contain two backslashes to much.