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.

src/agent.rs

@@ -498,7 +498,7 @@ impl AgentBuilder {
     /// WARNING: for 307 and 308 redirects, this value is ignored for methods that have a body.
     /// You must handle 307 redirects yourself when sending a PUT, POST, PATCH, or DELETE request.
     ///
-    /// ```
+    /// ```no_run
     /// # fn main() -> Result<(), ureq::Error> {
     /// # ureq::is_test(true);
     /// let result = ureq::builder()
@@ -535,7 +535,7 @@ impl AgentBuilder {
     /// Defaults to `ureq/[VERSION]`. You can override the user-agent on an individual request by
     /// setting the `User-Agent` header when constructing the request.
     ///
-    /// ```
+    /// ```no_run
     /// # #[cfg(feature = "json")]
     /// # fn main() -> Result<(), ureq::Error> {
     /// # ureq::is_test(true);

src/middleware.rs

@@ -24,7 +24,7 @@ use crate::{Error, Request, Response};
 ///
 /// A common use case is to add headers to the outgoing request. Here an example of how.
 ///
-/// ```
+/// ```no_run
 /// # #[cfg(feature = "json")]
 /// # fn main() -> Result<(), ureq::Error> {
 /// # use ureq::{Request, Response, MiddlewareNext, Error};
@@ -58,7 +58,7 @@ use crate::{Error, Request, Response};
 /// In the `examples` directory there is an additional example `count-bytes.rs` which uses
 /// a mutex lock like shown below.
 ///
-/// ```
+/// ```no_run
 /// # use ureq::{Request, Response, Middleware, MiddlewareNext, Error};
 /// # use std::sync::{Arc, Mutex};
 /// struct MyState {
@@ -89,7 +89,7 @@ use crate::{Error, Request, Response};
 /// This example shows how we can increase a counter for each request going
 /// through the agent.
 ///
-/// ```
+/// ```no_run
 /// # fn main() -> Result<(), ureq::Error> {
 /// # ureq::is_test(true);
 /// use ureq::{Request, Response, Middleware, MiddlewareNext, Error};