BUG #13160: Incorrect escaping of quotes in JSON export

The following bug has been logged on the website:

Bug reference:      13160
Logged by:          Andy
Email address:      me@andrewray.me
PostgreSQL version: 9.3.5
Operating system:   OSX
Description:

Postgres escapes quotes incorrectly when creating a JSON export.

    UPDATE models SET column='"hello"' WHERE id=1;

    COPY (SELECT row_to_json(models)
        FROM (SELECT column FROM shaders WHERE id=1) shaders)
        TO '/output.json';

The contents of output.json:

    {"column":"\\"hello\\""}

You can see that the quotes are escaped improperly and it creates invalid
JSON.
It should be:

    {"column":"\"hello\""}