Fix POST /api/v1/follows error when already following (#4878)

app/controllers/api/v1/follows_controller.rb

@@ -10,6 +10,12 @@ class Api::V1::FollowsController < Api::BaseController
     raise ActiveRecord::RecordNotFound if follow_params[:uri].blank?
 
     @account = FollowService.new.call(current_user.account, target_uri).try(:target_account)
+
+    if @account.nil?
+      username, domain = target_uri.split('@')
+      @account         = Account.find_remote!(username, domain)
+    end
+
     render json: @account, serializer: REST::AccountSerializer
   end