Set a default content type for JSON requests

When sending a JSON request a Content-Type of application/json is
usually wanted. This is often set as a default for JSON methods by HTTP
clients so can be confusing when it is not set. However, we do not want
to prevent the user from setting their own Content-Type.

src/request.rs

@@ -132,6 +132,9 @@ impl Request {
     /// ```
     #[cfg(feature = "json")]
     pub fn send_json(&mut self, data: SerdeValue) -> Response {
+        if let None = self.header("Content-Type") {
+            self.set("Content-Type", "application/json");
+        }
         self.do_call(Payload::JSON(data))
     }