PrefixMap type validation incomplete

[mdickinson]

The PrefixMap trait type turns some incorrect types (like list) into TraitErrors, but not others (like complex). Since the value of a PrefixMap trait is supposed to be a string, we should apply the same up-front validation as in BaseStr.

>>> from traits.api import *
>>> class A(HasTraits):
...     foo = PrefixMap({"yes": 1, "no": 0})
... 
>>> a = A()
>>> a.foo = "yes"
>>> a.foo = 1j
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/mdickinson/Enthought/ETS/traits/traits/trait_types.py", line 2911, in validate
    n = len(value)
TypeError: object of type 'complex' has no len()

[mdickinson]

Fixed in #956.