fix and simplify registry path traversal

[andre-merzky]

The important change is on the put() method where the line this = this[elem] needed to be de-indented. The original code failed when adding keys to an existing entry - the keys were placed in root instead.

[andre-merzky]

have a general wondering - should we consider something as the following case (speaking about reassignment and changing the type of a value if needed)

c = ru.zmq.RegistryClient(url=r.addr)

c.put('foo.bar.buz', 11)
c.put('foo.bar.buz.biz', 42)

which will raise an exception

.....
               this[leaf] = val
           TypeError: 'int' object does not support item assignment

but the following will pass as expected

c.put('foo.bar.buz', 11)
c.put('foo.bar.buz', {'biz': 42})

You are right, but I think the behavior is consistent: if a leaf is treated as a path element, an exception is appropriate. I don't think there is a good way to guess the user intention.

In the second case, the path is explicitly pruned and replaced by a dict, so that's all right in my book. The basically same behavior is what you would get on a standard dictionary:

#!/usr/bin/env python3

c = {'foo': {'bar': {'buz': None}}}

# this works
c['foo']['bar']['buz'] = 11
c['foo']['bar']['buz'] = {'biz': 42}

# this raises
c['foo']['bar']['buz'] = 11
c['foo']['bar']['buz']['biz'] = 42

$ python3 t.py
Traceback (most recent call last):
  File "t.py", line 12, in <module>
    c['foo']['bar']['buz']['biz'] = 42
TypeError: 'int' object does not support item assignment