fix(GuildMemberManager): options.roles on 'prune' (discordjs#4838)

src/managers/GuildMemberManager.js

@@ -162,19 +162,22 @@ class GuildMemberManager extends BaseManager {
    *    .then(pruned => console.log(`I just pruned ${pruned} people!`))
    *    .catch(console.error);
    */
-  prune({ days = 7, dry = false, count = true, roles = [], reason } = {}) {
+  prune({ days = 7, dry = false, count: compute_prune_count = true, roles = [], reason } = {}) {
     if (typeof days !== 'number') throw new TypeError('PRUNE_DAYS_TYPE');
 
-    const query = new URLSearchParams();
-    query.set('days', days);
-    query.set('compute_prune_count', count);
+    const query = { days };
+    const resolvedRoles = [];
 
     for (const role of roles) {
       const resolvedRole = this.guild.roles.resolveID(role);
       if (!resolvedRole) {
         return Promise.reject(new TypeError('INVALID_TYPE', 'roles', 'Array of Roles or Snowflakes', true));
       }
-      query.append('include_roles', role);
+      resolvedRoles.push(resolvedRole);
+    }
+
+    if (resolvedRoles.length) {
+      query.include_roles = dry ? resolvedRoles.join(',') : resolvedRoles;
     }
 
     const endpoint = this.client.api.guilds(this.guild.id).prune;
@@ -183,13 +186,12 @@ class GuildMemberManager extends BaseManager {
       return endpoint.get({ query, reason }).then(data => data.pruned);
     }
 
-    const body = [...query.entries()].reduce((acc, [k, v]) => {
-      if (k === 'include_roles') v = (acc[k] || []).concat(v);
-      acc[k] = v;
-      return acc;
-    }, {});
-
-    return endpoint.post({ data: body, reason }).then(data => data.pruned);
+    return endpoint
+      .post({
+        data: { ...query, compute_prune_count },
+        reason,
+      })
+      .then(data => data.pruned);
   }
 
   /**