Does not accept `:daily` as shift_age

[doloopwhile]

Logger.new fails if shift_age is a symbol.

Error:

irb(main):003:0> Logger.new('a.log', :daily)
ArgumentError (invalid :shift_age :daily, should be daily, weekly, monthly, or everytime)

Environment:

$ bundle list
Gems included by the bundle:
  * bundler (2.0.2)
  * logger (1.4.2)

Expected behavior (My suggestion for design change):

Logger.new('a.log', :daily) works as well as Logger.new('a.log', 'daily').

[skryukov]

@sonots seems like a developer-friendly addition. What are your thoughts on this?

[hsbt]

Fixed at https://github.com/ruby/logger/commit/83502c2107c282e9e072865ab55252f2b37ec779