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.
Merge pull request #23 from kanru/rfc1123

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