Support honoring Retry-After headers for 429, 503 and 301 in Async Methods

[ryber]

Is your feature request related to a problem? Please describe. Support honoring Retry-After headers for 429, 503 and 301

Describe the solution you'd like When a server issues a 429, 503 or 301 with a Retry-After header the client is asked to wait that period (either a date, or a number of seconds). Enable Unirest to do this automatically

Questions to consider:

• do we roll this out as default behavior or make users enable it?
• do we have a max number of times to retry before giving up? (do it 3 times and then give up if we are still getting the header)
• do we have a max time before giving up? (if the server tells us to wait until 2023?)

[ryber]

this is done for the regular requests. Apache's async methods really don't like making a request inside of making a request.

[ryber]

complete