title: 'Asset' object has no attribute 'startswith' when using avatar_url with create_webhook
body: <p>I have a command that creates a webhook, creating the webhook and name works find but when i add <code>avatar</code> and set it to the avatar url of the person mentioned I get error: <code>'Asset' object has no attribute 'startswith'</code> I think i may have to use <code>with open</code> or anything like that to open the image from my pc which i dodn't want, i want it to be the image of the mentioned person not an image file from my Pc, anyway to do this?</p>
  
  <pre><code>@bot.command()
  async def whook(ctx,user:discord.Member):
      await ctx.channel.create_webhook(name="Mr. W.Hook",avatar=user.avatar_url)
      print("Created")
  </code></pre>