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
Add constant to Enumerable for testable methods
Bump enumerable cursor spec test up