Update some more doctests.

I missed these in my previous doctest PR.

The doctests all now run without accessing the network. Tested by
turning off networking and running them.

Request.call's doctest wouldn't run because it relied on making a custom
AgentBuilder and building it, which bypasses the test_agent. I concluded
that this doctest was mostly illustrating behavior of AgentBuilder, not
call(), and simplified it to be more like the other calling methods on
request.

src/testserver.rs

@@ -22,6 +22,10 @@ pub(crate) fn test_agent() -> Agent {
             stream.write_all(b"Content-Length: 100\r\n")?;
             stream.write_all(b"\r\n")?;
             stream.write_all(&[0; 100])?;
+        } else if headers.path() == "/hello_world.json" {
+            stream.write_all(b"HTTP/1.1 200 OK\r\n")?;
+            stream.write_all(b"\r\n")?;
+            stream.write_all(br#"{"hello": "world"}"#)?;
         } else if headers.path() == "/redirect/3" {
             stream.write_all(b"HTTP/1.1 302 Found\r\n")?;
             stream.write_all(b"Location: /redirect/3\r\n")?;