add DAY_NAMES constant instead of ::Time::RFC2822_DAY_NAME

[mpg-yuya-tani]

Fixes #211

before

$ ruby -v
ruby 3.1.0preview1 (2021-11-09 master 5a3b2e6141) [x86_64-darwin20]
$ bundle exec rake
EEEEEEEEEEEEEEEEEE.E.E..E..EEEEEEEEEEEEEEEEEEE...EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE......EEEEEEEEEEEEEEEEEEEEEEEEEEEEEE.......EEEEEEEEEEEEEEEEEEEEEEEE........EEEEEEEEEEEEEEEE.E......EEEEEEEEEEEEEEEEEEEEEEEE.EEEEEEEEEE...

  1) Error:
time extensions#test_0007_calculate business time between different times on the same date (counter clockwise):
NameError: uninitialized constant Time::RFC2822_DAY_NAME

        ::Time::RFC2822_DAY_NAME.map(&:downcase).map(&:to_sym)[num]
              ^^^^^^^^^^^^^^^^^^
    /Users/yuya.tani/Work/8.oss/business_time/lib/business_time/config.rb:195:in `int_to_wday'
    /Users/yuya.tani/Work/8.oss/business_time/lib/business_time/config.rb:124:in `beginning_of_workday'
    /Users/yuya.tani/Work/8.oss/business_time/lib/business_time/time_extensions.rb:29:in `beginning_of_workday'
    /Users/yuya.tani/Work/8.oss/business_time/lib/business_time/time_extensions.rb:47:in `before_business_hours?'
    /Users/yuya.tani/Work/8.oss/business_time/lib/business_time/time_extensions.rb:58:in `roll_forward'
    /Users/yuya.tani/Work/8.oss/business_time/lib/business_time/time_extensions.rb:152:in `business_time_until'
    /Users/yuya.tani/Work/8.oss/business_time/test/test_time_extensions.rb:51:in `block (2 levels) in <top (required)>'
・
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247 runs, 130 assertions, 0 failures, 206 errors, 0 skips

after

$ ruby -v
ruby 3.1.0preview1 (2021-11-09 master 5a3b2e6141) [x86_64-darwin20]
$ bundle exec rake
............................................................................................................................................................................................................
...........................................

Finished in 0.247837s, 996.6228 runs/s, 1557.4753 assertions/s.

247 runs, 386 assertions, 0 failures, 0 errors, 0 skips

[rmm5t]

Thank you very much!