Remove error_on_non_2xx. (#272)

After the recent changes in #257, it's probably not necessary. It's now
quite easy to use a match statement to extract responses for certain
status codes, or all status codes.

Add documentation on how to turn a status code error back into a
Response.

src/request.rs

@@ -120,7 +120,7 @@ impl Request {
         let unit = Unit::new(&self.agent, &self.method, &url, &self.headers, &reader);
         let response = unit::connect(unit, true, reader, None).map_err(|e| e.url(url.clone()))?;
 
-        if self.error_on_non_2xx && response.status() >= 400 {
+        if response.status() >= 400 {
             Err(Error::Status(response.status(), response))
         } else {
             Ok(response)
@@ -336,26 +336,6 @@ impl Request {
             .push((param.to_string(), value.to_string()));
         self
     }
-
-    /// By default, if a response's status is anything but a 2xx or 3xx,
-    /// call()/send() and related methods will return an Error. If you want
-    /// to handle such responses as non-errors, set this to `false`.
-    ///
-    /// Example:
-    /// ```
-    /// # fn main() -> Result<(), ureq::Error> {
-    /// # ureq::is_test(true);
-    /// let response = ureq::get("http://httpbin.org/status/500")
-    ///     .error_on_non_2xx(false)
-    ///     .call()?;
-    /// assert_eq!(response.status(), 500);
-    /// # Ok(())
-    /// # }
-    /// ```
-    pub fn error_on_non_2xx(mut self, value: bool) -> Self {
-        self.error_on_non_2xx = value;
-        self
-    }
 }
 
 #[test]