Gets rid of synthetic_error, and makes the various send_* methods return `Result<Response, Error>`.
Introduces a new error type "HTTP", which represents an error due to status codes 4xx or 5xx.
The HTTP error type contains a boxed Response, so users can read the actual response if they want.
Adds an `error_for_status` setting to disable the functionality of treating 4xx and 5xx as errors.
Adds .unwrap() to a lot of tests.
Fixes#128.
This removes the necessity to take the result of Response::into_json and
having to convert it into a struct by using serde_json::from_value
This adds no new dependencies since serde_json already depends on serde.
Users of ureq will have to include `serde_derive` either by importing it
directly or by using serde with the `derive` feature, unless they want to
manually implement `Deserialize` on their structs.
