Fix store_attr to preserve existing instance attributes

[erikgaas]

Fixes #496

store_attr now checks if an attribute already exists on self before storing, preserving any modifications made before calling store_attr().

Before:

class T_:
    def __init__(self, a, b, c):
        self.a = a - 1
        store_attr()  # self.a gets overwritten back to original 'a'

After:

class T_:
    def __init__(self, a, b, c):
        self.a = a - 1
        store_attr()  # self.a stays as a - 1

The fix uses getattr(self, n, fr.f_locals[n]) to prefer existing attribute values over the original arguments.