Add 'wait' parameter to http::execute_webhook

The 'wait' parameter allows you to specify waiting for the Message to be
sent prior to receiving a response, which will have Discord include the
JSON representation of the Message in the body.

src/builder/execute_webhook.rs

@@ -44,7 +44,7 @@ use ::internal::prelude::*;
 ///         .name("Rust by Example")
 ///         .value("A collection of Rust examples on topics, useable in-browser")));
 ///
-/// let _ = webhook.execute(|w| w
+/// let _ = webhook.execute(false, |w| w
 ///     .content("Here's some information on Rust:")
 ///     .embeds(vec![website, resources]));
 /// ```
@@ -69,7 +69,7 @@ impl ExecuteWebhook {
     /// #
     /// let avatar_url = "https://i.imgur.com/KTs6whd.jpg";
     ///
-    /// let _ = webhook.execute(|w| w
+    /// let _ = webhook.execute(false, |w| w
     ///     .avatar_url(avatar_url)
     ///     .content("Here's a webhook"));
     /// ```
@@ -93,7 +93,7 @@ impl ExecuteWebhook {
     /// #
     /// # let webhook = rest::get_webhook_with_token(0, "").unwrap();
     /// #
-    /// if let Err(why) = webhook.execute(|w| w.content("foo")) {
+    /// if let Err(why) = webhook.execute(false, |w| w.content("foo")) {
     ///     println!("Err sending webhook: {:?}", why);
     /// }
     /// ```
@@ -135,7 +135,7 @@ impl ExecuteWebhook {
     /// #
     /// # let webhook = rest::get_webhook_with_token(0, "").unwrap();
     /// #
-    /// if let Err(why) = webhook.execute(|w| w.content("hello").tts(true)) {
+    /// if let Err(why) = webhook.execute(false, |w| w.content("hello").tts(true)) {
     ///     println!("Err sending webhook: {:?}", why);
     /// }
     /// ```
@@ -156,7 +156,7 @@ impl ExecuteWebhook {
     /// #
     /// # let webhook = rest::get_webhook_with_token(0, "").unwrap();
     /// #
-    /// if let Err(why) = webhook.execute(|w| w.content("hello").username("hakase")) {
+    /// if let Err(why) = webhook.execute(false, |w| w.content("hello").username("hakase")) {
     ///     println!("Err sending webhook: {:?}", why);
     /// }
     /// ```

src/http/mod.rs

@@ -37,7 +37,9 @@ use hyper::client::{
     Response as HyperResponse,
     Request,
 };
+use hyper::header::ContentType;
 use hyper::method::Method;
+use hyper::mime::{Mime, SubLevel, TopLevel};
 use hyper::net::HttpsConnector;
 use hyper::{Error as HyperError, Result as HyperResult, Url, header};
 use hyper_native_tls::NativeTlsClient;
@@ -803,28 +805,41 @@ pub fn edit_webhook_with_token(webhook_id: u64, token: &str, map: &JsonMap) -> R
 /// let token = "ig5AO-wdVWpCBtUUMxmgsWryqgsW3DChbKYOINftJ4DCrUbnkedoYZD0VOH1QLr-S3sV";
 /// let map = ObjectBuilder::new().insert("content", "test").build();
 ///
-/// let message = match http::execute_webhook(id, token, map) {
-///     Ok(message) => message,
+/// let message = match http::execute_webhook(id, token, true, map) {
+///     Ok(Some(message)) => message,
+///     Ok(None) => {
+///         println!("Expected a webhook message");
+///
+///         return;
+///     },
 ///     Err(why) => {
 ///         println!("Error executing webhook: {:?}", why);
 ///
 ///         return;
 ///     },
 /// };
+/// ```
 ///
 /// [`Channel`]: ../model/enum.Channel.html
 /// [`Message`]: ../model/struct.Message.html
 /// [Discord docs]: https://discordapp.com/developers/docs/resources/webhook#querystring-params
-pub fn execute_webhook(webhook_id: u64, token: &str, map: &JsonMap) -> Result<Message> {
+pub fn execute_webhook(webhook_id: u64, token: &str, wait: bool, map: &JsonMap)
+    -> Result<Option<Message>> {
     let body = serde_json::to_string(map)?;
+
     let client = request_client!();
 
     let response = retry(|| client
-        .post(&format!(api!("/webhooks/{}/{}"), webhook_id, token))
-        .body(&body))
+        .post(&format!(api!("/webhooks/{}/{}?wait={}"), webhook_id, token, wait))
+        .body(&body)
+        .header(ContentType(Mime(TopLevel::Application, SubLevel::Json, vec![]))))
         .map_err(Error::Hyper)?;
 
-    serde_json::from_reader::<HyperResponse, Message>(response).map_err(From::from)
+    if response.status == StatusCode::NoContent {
+        return Ok(None);
+    }
+
+    serde_json::from_reader::<HyperResponse, Message>(response).map(Some).map_err(From::from)
 }
 
 /// Gets the active maintenances from Discord's Status API.

src/model/webhook.rs

@@ -148,7 +148,7 @@ impl Webhook {
     /// let mut webhook = http::get_webhook_with_token(id, token)
     ///     .expect("valid webhook");
     ///
-    /// let _ = webhook.execute(|w| w.content("test")).expect("Error executing");
+    /// let _ = webhook.execute(false, |w| w.content("test")).expect("Error executing");
     /// ```
     ///
     /// Execute a webhook with message content of `test`, overriding the
@@ -171,15 +171,16 @@ impl Webhook {
     ///                   thread safety.")
     ///     .url("https://rust-lang.org"));
     ///
-    /// let _ = webhook.execute(|w| w
+    /// let _ = webhook.execute(false, |w| w
     ///     .content("test")
     ///     .username("serenity")
     ///     .embeds(vec![embed]))
     ///     .expect("Error executing");
     /// ```
     #[inline]
-    pub fn execute<F: FnOnce(ExecuteWebhook) -> ExecuteWebhook>(&self, f: F) -> Result<Message> {
-        http::execute_webhook(self.id.0, &self.token, &f(ExecuteWebhook::default()).0)
+    pub fn execute<F: FnOnce(ExecuteWebhook) -> ExecuteWebhook>(&self, wait: bool, f: F)
+        -> Result<Option<Message>> {
+        http::execute_webhook(self.id.0, &self.token, wait, &f(ExecuteWebhook::default()).0)
     }
 
     /// Retrieves the latest information about the webhook, editing the