Fix error raised when using the roles parameter.

discord/guild.py

@@ -1379,7 +1379,7 @@ class Guild(Hashable):
             raise InvalidArgument('Expected int for ``days``, received {0.__class__.__name__} instead.'.format(days))
 
         if roles:
-            roles = [role.id for role in roles]
+            roles = [str(role.id) for role in roles]
 
         data = await self._state.http.prune_members(self.id, days, compute_prune_count=compute_prune_count, roles=roles, reason=reason)
         return data['pruned']

discord/http.py

@@ -660,12 +660,12 @@ class HTTPClient:
         return self.request(Route('GET', '/guilds/{guild_id}/members/{member_id}', guild_id=guild_id, member_id=member_id))
 
     def prune_members(self, guild_id, days, compute_prune_count, roles, *, reason=None):
-        params = {
+        payload = {
             'days': days,
             'compute_prune_count': 'true' if compute_prune_count else 'false',
-            'include_roles': roles
+            'include_roles': ', '.join(roles)
         }
-        return self.request(Route('POST', '/guilds/{guild_id}/prune', guild_id=guild_id), params=params, reason=reason)
+        return self.request(Route('POST', '/guilds/{guild_id}/prune', guild_id=guild_id), json=payload, reason=reason)
 
     def estimate_pruned_members(self, guild_id, days):
         params = {