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.
When sending a JSON request a Content-Type of application/json is
usually wanted. This is often set as a default for JSON methods by HTTP
clients so can be confusing when it is not set. However, we do not want
to prevent the user from setting their own Content-Type.
