Clarify correct specification of dependency on serde (#525)

Closes #494

src/async_impl/request.rs

@@ -428,6 +428,7 @@ pub(crate) fn replace_headers(dst: &mut HeaderMap, src: HeaderMap) {
 mod tests {
     use super::Client;
     use std::collections::BTreeMap;
+    use serde::Serialize;
 
     #[test]
     fn add_query_append() {

src/error.rs

@@ -9,10 +9,11 @@ use {StatusCode, Url};
 /// # Examples
 ///
 /// ```
-/// #[macro_use]
-/// extern crate serde_derive;
+/// extern crate serde;
 /// extern crate reqwest;
 ///
+/// use serde::Deserialize;
+///
 /// #[derive(Deserialize)]
 /// struct Simple {
 ///    key: String

src/lib.rs

@@ -193,9 +193,6 @@ extern crate mime_guess;
 #[cfg(feature = "default-tls")]
 extern crate native_tls;
 extern crate serde;
-#[cfg(test)]
-#[macro_use]
-extern crate serde_derive;
 extern crate serde_json;
 extern crate serde_urlencoded;
 extern crate time;

src/request.rs

@@ -582,6 +582,7 @@ mod tests {
     use {body, Client, Method};
     use header::{ACCEPT, HOST, HeaderMap, HeaderValue, CONTENT_TYPE};
     use std::collections::{BTreeMap, HashMap};
+    use serde::Serialize;
     use serde_json;
     use serde_urlencoded;
 

src/response.rs

@@ -186,9 +186,10 @@ impl Response {
     ///
     /// ```rust
     /// # extern crate reqwest;
-    /// # #[macro_use] extern crate serde_derive;
+    /// # extern crate serde;
     /// #
     /// # use reqwest::Error;
+    /// # use serde::Deserialize;
     /// #
     /// #[derive(Deserialize)]
     /// struct Ip {