temporarily add no_run to middleware doctests (#626)

These test were failing because they made live requests to httpbin.org,
which is serving intermittent 500s.

In general we strive to make our test and doctests interact with a local
test server rather than a live website. For doctests we do this with
some sleight of hand in a hidden `ureq::is_test(true)`, which swaps out
the default agent returned by ureq::agent. However, since the middleware
doctests rely on constructing a custom agent with the middleware, they
can't rely on this.

I suspect we can get around this problem but it may take some thinking.

Similarly disable the AgentBuilder::user_agent and redirects doctests
and the agent_set_header unittest.

tests/https-agent.rs

@@ -1,27 +1,28 @@
 #[cfg(all(feature = "json", any(feature = "tls", feature = "tls-native")))]
-#[test]
-fn agent_set_header() {
-    use serde::Deserialize;
-    use std::collections::HashMap;
+// test temporarily disabled because httpbin is down / we need to figure out
+// how to eliminate the external dependency.
+// #[test]
+// fn agent_set_header() {
+//     use serde::Deserialize;
+//     use std::collections::HashMap;
 
-    #[derive(Deserialize, Debug)]
-    struct HttpBin {
-        headers: HashMap<String, String>,
-    }
-
-    let agent = ureq::Agent::new();
-    let resp = agent
-        .get("https://httpbin.org/get")
-        .set("header", "value")
-        .set("Connection", "close")
-        .call()
-        .unwrap();
-    assert_eq!(resp.status(), 200);
-    let json: HttpBin = resp.into_json().unwrap();
-    // println!("{:?}", json);
-    assert_eq!("value", json.headers.get("Header").unwrap());
-}
+//     #[derive(Deserialize, Debug)]
+//     struct HttpBin {
+//         headers: HashMap<String, String>,
+//     }
 
+//     let agent = ureq::Agent::new();
+//     let resp = agent
+//         .get("https://httpbin.org/get")
+//         .set("header", "value")
+//         .set("Connection", "close")
+//         .call()
+//         .unwrap();
+//     assert_eq!(resp.status(), 200);
+//     let json: HttpBin = resp.into_json().unwrap();
+//     // println!("{:?}", json);
+//     assert_eq!("value", json.headers.get("Header").unwrap());
+// }
 #[test]
 #[cfg(any(feature = "tls", feature = "tls-native"))]
 // From here https://badssl.com/download/