_normalize_string_argument should send the dict "values" not the "keys" to the _check_types function

[ste-lau]

Hi it's me again :)
Just caught a small bug and the fix is very simple (see below).

Symptom:

When trying to supply a dict for the node_shape argument (so that different nodes can have different shapes) it currently throws an exception:

 _main.py", line 1167, in _check_types
  raise TypeError(msg)
  TypeError: Item 0 in node_shape is of the wrong type.
     Expected type: <class 'str'>
     Actual type: <class 'int'>

Cause:

The first arg that currently is sent to the _check types function is str_or_dict which then gets iterated over as "for item in items". So it it's a dict, it's the keys which get iterated over rather than the values .

Fix:

The first arg that is sent to the **_check type**s function should be str_or_dict.values() instead of the current str_or_dict (as it correctly is in _normalize_numeric_argument)

[paulbrodersen]

You are becoming my favourite person on github! If you add an email to your github account, then I can attribute your fixes to your account such that you are listed as one of the contributors.