rfc1123 is wrongly formatted by toString

```
What steps will reproduce the problem?
1. Date.parse('Thu Jan 20 19:49:15 +0000 2011').toString('r')

What is the expected output? What do you see instead?

Expected: Wed, 20 Apr 2011 19:49:15 GMT
Obtained: Wed, 20 Apr 2011 19:49:15 G4T

What version of the product are you using? On what operating system?

SVN. Linux + Firefox 4.

Please provide any additional information below.

toString uses a regex to replace placeholders in the given format. rfc1123
format (param 'r' passed to toString) contains a trailing 'GMT', which 'M' is
erroneously matched as month, and replaced by the month number, rendering a
wrong data.

Many solutions can be applied:

- modify the regex to ignore 'GMT'.
- remove 'GMT' from the rfc1123 format string, and prepend it to the resulting
formatted string before returning it.

```

Original issue reported on code.google.com by `aitorc...@gmail.com` on 4 Apr 2011 at 8:27