Fix check `destination` is a directory

it's a fabfile.py:

```python
from fabric.contrib.files import upload_template

context = {
    'name': 'oscarmlage',
}

def update_temp():
    upload_template(filename='xmas.conf',
                    destination='/home/vagrant/test/tmp/',
                    context=context,
                    template_dir='.',
                    use_jinja=True)
```

when i execute it:

```python
➜  test  fab -H localhost update_temp --colorize-errors 
[localhost] Executing task 'update_temp'
[localhost] put: <file obj> -> /home/vagrant/test/tmp/

Fatal error: put() encountered an exception while uploading '<StringIO.StringIO instance at 0x804415f80>'

Underlying exception:
    Failure

Aborting.
Disconnecting from localhost... done.
put() encountered an exception while uploading '<StringIO.StringIO instance at 0x804415f80>'

Underlying exception:
    Failure

➜  test  ipython
In [1]: import os

In [2]: os.path.isdir('/home/vagrant/test/tmp/')
Out[2]: True
```