Add `each_page` enumerable method and consolidate tests

It is useful to handle the results of a single api call directly before
another api call is made for more. The `each_page` method provides an
interface for this.
Add method for yielding the page results and cursor

This lets the user process the results of an api request and ask the
cursor for information.
Delegate cursor rate_limit to the request

This also ensures the cursor has the latest request and rate limit
information. I was hoping to use rails' nifty delegators:

  delegate :rate_limit, to: :@request

But sadly this requires ActiveSupport
Use let instead of subject for enumerable rspec tests
Use symbol for content type in http header