Expression trait doesn't store default mapped value correctly

[corranwebster]

For example, see how the default value for e_ is a string, but is a code object for a non-default value.

In [1]: from traits.api import Expression, HasTraits                            
In [2]: class Test(HasTraits): 
   ...:     e = Expression("3") 
   ...:                                                                         
In [3]: t = Test()                                                              
In [4]: t.e
Out[4]: '3'
In [5]: t.e_                                                                    
Out[5]: '3'
In [6]: t.e = '4'                                                               
In [7]: t.e                                                                     
Out[7]: '4'
In [8]: t.e_                                                                   
Out[8]: <code object <module> at 0x7ffb01cbec90, file "<string>", line 1>

Other mapped traits like Map seem to do things right.

This is related to #1114 and would be likely fixed by the suggestion there.