BUG #6453: auto_explain produces bad json output

The following bug has been logged on the website:

Bug reference:      6453
Logged by:          Daniele Varrazzo
Email address:      daniele.varrazzo@gmail.com
PostgreSQL version: 9.1.2
Operating system:   Any
Description:=20=20=20=20=20=20=20=20

Bug verified on PG 9.0.5 and 9.1.2.

auto_explain in json format produces an output such as:

        [
          "Query Text": "select pg_sleep(2);",
          "Plan": {
            "Node Type": "Result",
            [...]
            "Plan Width": 0
          }
        ]

The representation is wrapped in a pair of [], but the content is a mapping:
it should have been a pair of {}.