Keep RFC and RSS strings in English regardless of the locale

[PDGGK]

After pendulum.set_locale(...), the RFC 822/850/1036/1123/2822 and RSS string methods use the locale's day and month names, which email, HTTP and RSS parsers reject:

import email.utils, pendulum

dt = pendulum.datetime(1975, 12, 25, 14, 15, 16)
pendulum.set_locale("fr")
dt.to_rfc2822_string()  # 'jeu., 25 déc. 1975 14:15:16 +0000'
email.utils.parsedate_to_datetime(dt.to_rfc2822_string())
# ValueError: Invalid date value or format "jeu., 25 déc. 1975 14:15:16 +0000"

These formats are protocol formats with fixed English names, and the docs show them in English. to_cookie_string() and to_day_datetime_string() already pass locale="en"; this makes _to_string default to English so the other common formats behave the same. format() keeps using the current locale.

With the change, to_rfc2822_string() returns Thu, 25 Dec 1975 14:15:16 +0000, which email.utils parses.

• Added tests for changed code (test_common_formats_are_not_localized, which fails for the six affected methods without the change).
• Updated documentation for changed code (no change needed; the docs already show English output).