Connection.rb: Allow string and empty response body

[jasonopslevel]

There are certain responses that only respond with a status code, and an empty body like all of the DELETE requests. For example: delete_webhook.

This change makes parsing the body (in particular, ParseTimeStrings) more permissive, and doesn't raise for valid requests:

connection = PagerDuty::Connection.new(token, token_type: :Bearer)
id = "PXXXXX"
connection.delete("webhook_subscriptions/#{id}")

# before
= >  Can't parse times of String:  (RuntimeError)

# after
 => ""