Fix the watch prefix bug

The watch API cannot work properly. When you set up a key prefix,
it will still watch the whole key space. This commit fixes it.

The reason is that when it deals with the original key prefix string,
it double encodes the string in client.py and watch.py.

etcd3gw/client.py

@@ -366,7 +366,7 @@ def iterator():
     def watch_prefix(self, key_prefix, **kwargs):
         """The same as ``watch``, but watches a range of keys with a prefix."""
         kwargs['range_end'] = \
-            _increment_last_byte(_encode(key_prefix))
+            _increment_last_byte(key_prefix)
         return self.watch(key_prefix, **kwargs)
 
     def watch_once(self, key, timeout=None, **kwargs):
@@ -392,7 +392,7 @@ def callback(event):
     def watch_prefix_once(self, key_prefix, timeout=None, **kwargs):
         """Watches a range of keys with a prefix, similar to watch_once"""
         kwargs['range_end'] = \
-            _increment_last_byte(_encode(key_prefix))
+            _increment_last_byte(key_prefix)
         return self.watch_once(key_prefix, timeout=timeout, **kwargs)