Change +rfc-1123-format+ to use numeric timezone indicators

RFC-1123 section 5.2.14 writes

> "There is a strong trend towards the use of numeric timezone
indicators, and implementations SHOULD use numeric timezones
instead of timezone names."

RFC-822 section 5.1 writes the syntax of timezone

```
zone        =  "UT"  / "GMT"                ; Universal Time
                                            ; North American : UT
            /  "EST" / "EDT"                ;  Eastern:  - 5/ - 4
            /  "CST" / "CDT"                ;  Central:  - 6/ - 5
            /  "MST" / "MDT"                ;  Mountain: - 7/ - 6
            /  "PST" / "PDT"                ;  Pacific:  - 8/ - 7
            /  1ALPHA                       ; Military: Z = UT;
                                            ;  A:-1; (J not used)
                                            ;  M:-12; N:+1; Y:+12
            / ( ("+" / "-") 4DIGIT )        ; Local differential
                                            ;  hours+min. (HHMM)
```

Which I feel using the numeric is the way to go. It's always valid
that we don't have to worry about if the timezone abbreviation is in
the allowed list.