Imposible to download file into directory
## Context

* Operating System: Arch Linux
* Python Version: 3.7.3
* aiogram version: 2.2
* aiohttp version: 3.5.4

## Expected Behavior

Able to download file into specified directory.

## Current Behavior

Got `IsADirectoryError` exception.

## Failure Information (for bugs)

Problem lay in line https://github.com/aiogram/aiogram/blob/dev-2.x/aiogram/types/mixins.py#L27:
```
os.path.join(destination, file.file_path)
```
which is noop in current context.

### Steps to Reproduce

1. Setup message handler:
```
async def new_photo(message: types.Message):
    await message.photo[0].download(destination='/path/to/existing/directory')
```
2. Send photo to bot.
3. Get tracelog with `IsADirectoryError` exception.
diff --git a/aiogram/types/mixins.py b/aiogram/types/mixins.py
index f11a1760..13f8412f 100644
--- a/aiogram/types/mixins.py
+++ b/aiogram/types/mixins.py
@@ -24,7 +24,7 @@ class Downloadable:
         if destination is None:
             destination = file.file_path
         elif isinstance(destination, (str, pathlib.Path)) and os.path.isdir(destination):
-            os.path.join(destination, file.file_path)
+            destination = os.path.join(destination, file.file_path)
         else:
             is_path = False
 
['aiogram/types/mixins.py']