Fix guild leaving result

When leaving a guild, `rest::leave_guild` would attempt to deserialize a
JSON body containing a partial amount of guild info, resulting in an
error:

```
Could not leave guild: Json(ErrorImpl { code: EofWhileParsingValue, line: 1, column: 0 })
```

Although the bot would still actually leave the guild, it would always
return this error.

To fix this, don't try to deserialize anything and only check for a 204
instead.

src/client/rest/mod.rs

@@ -1341,13 +1341,8 @@ pub fn leave_group(guild_id: u64) -> Result<Group> {
 }
 
 /// Leaves a guild.
-pub fn leave_guild(guild_id: u64) -> Result<PartialGuild> {
-    let response = request!(Route::UsersMeGuildsId,
-                            delete,
-                            "/users/@me/guilds/{}",
-                            guild_id);
-
-    serde_json::from_reader::<HyperResponse, PartialGuild>(response).map_err(From::from)
+pub fn leave_guild(guild_id: u64) -> Result<()> {
+    verify(204, request!(Route::UsersMeGuildsId, delete, "/users/@me/guilds/{}", guild_id))
 }
 
 /// Deletes a user from group DM.

src/model/guild/guild_id.rs

@@ -356,7 +356,7 @@ impl GuildId {
 
     /// Leaves the guild.
     #[inline]
-    pub fn leave(&self) -> Result<PartialGuild> {
+    pub fn leave(&self) -> Result<()> {
         rest::leave_guild(self.0)
     }
 

src/model/guild/mod.rs

@@ -642,7 +642,7 @@ impl Guild {
 
     /// Leaves the guild.
     #[inline]
-    pub fn leave(&self) -> Result<PartialGuild> {
+    pub fn leave(&self) -> Result<()> {
         self.id.leave()
     }
 

src/model/guild/partial_guild.rs

@@ -350,7 +350,7 @@ impl PartialGuild {
 
     /// Leaves the guild.
     #[inline]
-    pub fn leave(&self) -> Result<PartialGuild> {
+    pub fn leave(&self) -> Result<()> {
         self.id.leave()
     }