Switch to Result-based API. (#132)

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.

tests/https-agent.rs

@@ -1,18 +1,3 @@
-#[cfg(all(test, any(feature = "tls", feature = "native-tls")))]
-use std::io::Read;
-
-#[cfg(any(feature = "tls", feature = "native-tls"))]
-#[test]
-fn tls_connection_close() {
-    let agent = ureq::Agent::default().build();
-    let resp = agent
-        .get("https://example.com/404")
-        .set("Connection", "close")
-        .call();
-    assert_eq!(resp.status(), 404);
-    resp.into_reader().read_to_end(&mut vec![]).unwrap();
-}
-
 #[cfg(feature = "tls")]
 #[cfg(feature = "cookies")]
 #[cfg(feature = "json")]
@@ -35,7 +20,8 @@ fn agent_set_cookie() {
     let resp = agent
         .get("https://httpbin.org/get")
         .set("Connection", "close")
-        .call();
+        .call()
+        .unwrap();
     assert_eq!(resp.status(), 200);
     assert_eq!(
         "name=value",
@@ -133,7 +119,8 @@ fn tls_client_certificate() {
     let resp = agent
         .get("https://client.badssl.com/")
         .set_tls_config(std::sync::Arc::new(tls_config))
-        .call();
+        .call()
+        .unwrap();
 
     assert_eq!(resp.status(), 200);
 }