Add Accept-Encoding to CORS headers.

- Also ensure the tests pick up all of the headers and correctly communicate what is missing.
Merge pull request #11 from ohookins/add_accept_encoding_to_cors

Add Accept-Encoding to CORS headers.
Example outline of RoundTripper with injected logic
Implementations of Delayers and Retryers
Added more tests, custom error structs and fixed logic in essential parts: For this introduced a tristate as decision output of retrier aspecifiedd evaluator for composed strategies
renaming Yes,No,Maybe to Retry, Ignore, Abort
update comment
Constrain Retryer interface for simpler logic

In order to reduce the logic and state needed from the Retryer
interface, limit errors only to the Abort Decision.  This simplifies
composition, short circuiting Aborts with errors and preferring Retry
over Ignore.

Removes the exported fields for the exported errors - treating them only
as types for type assertions.

Removes expectations that Retry and Ignore may return errors.

Rename RetryBy to All because the package name is included in the
callsite like `retry.All(...)`.

Uses Ignore as the zero value for a Decision, made explicit at
initialization.
Formulate documentation

Avoids confusing multiple "attempts" of Retryers with multiple
"Attempt"s of RoundTrip.
Merge pull request #13 from streadway/retry

Alternate Retry Transport

Closes #14
Closes #12
close the response body and constrains RoundTrip implementation to spec (return nil response when returning error)
custom errors export status code of last attempt
improved loop, removed status code from custom error, inverse branches in Attempt.StatusCode()
remove Attempt.StatusCode convenience function
Merge pull request #15 from tchakabam/close-response-and-obey-roundtripper-spec

close the response body and constrains RoundTrip implementation to spec