Default :keep_assets to nil to eliminate cap-doctor warning.

[JasonPoll]

Summary

First mentioned in #251, then once again in #262, when setting a value for :keep_assets, cap doctor will emit a warning:

# deploy.rb
set :keep_assets, -> { fetch(:keep_releases, 5) }

cap staging doctor 
# ...
# ....snipped for brevity
    :keep_assets is not a recognized Capistrano setting (/Users/jason.poll/.rbenv/versions/3.1.2/lib/ruby/3.1.0/delegate.rb:87)

Using:

• ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [arm64-darwin21]
• capistrano (3.17.3)
• capistrano-rails (1.6.2)

[mattbrictson]

Looks great! Thanks for the contribution!