Correction exists

Hello,
The original problem can occur with custom ids. Specialy with ids wich contains characters must be url encoded. For example an url.
To reproduce this issue follow the following steps

```python
c = ES('localhost:9200')
myDoc = { "field" : "content"}
c.index(myDoc, 'myindex', 'mydoctype', id='http://example.com')
c.get('myindex', 'mydoctype', 'http://example.com')
#return the object
c.exists('myindex', 'mydoctype', 'http://example.com')
#in the latest version return false with pies 0.20.
```