Explicitly require date for DateTime

[nickpresta]

Description

This change fixes an issue where it seems Farday is using DateTime, specifically for retry functionality, without requiring it. This leads to a NameError. Without explicitly requiring date, retry functionality fails with the following error:

NameError: uninitialized constant Faraday::Request::Retry::DateTime
from /Users/npresta/.rbenv/versions/2.4.4/lib/ruby/gems/2.4.0/gems/faraday-0.15.4/lib/faraday/request/retry.rb:194:in `calculate_retry_after'
Caused by Faraday::RetriableResponse:
from /Users/npresta/.rbenv/versions/2.4.4/lib/ruby/gems/2.4.0/gems/faraday-0.15.4/lib/faraday/request/retry.rb:129:in `block in call'

It looks like this was introduced in 6ddf9c3 Perhaps the testing environment, or IRB, automatically requires date, but if you just run this in Ruby directly:

$ ruby -e 'DateTime.rfc2822("hi")'
-e:1:in `<main>': uninitialized constant DateTime (NameError)

Ruby version: 2.4.4

Additional Notes

This seems like a straightforward bugfix, but likely doesn't impact many people (no open issues, no Google searches) so I would imagine most Ruby apps have date imported somewhere.

[iMacTia]

Ah good catch!
This has probably impacted few people because Ruby Frameworks (e.g. Rails, Sinatra) will include that for you.
Anyway it's a Ruby core library and doesn't require any external gem, so this change shouldn't break anyone's code.