Copy non-callable Optional defaults so mutable defaults aren't shared (fixes #352)

[vineethsaivs]

Fixes #352.

A non-callable default on an Optional (e.g. default=[] or default={}) was inserted into every validation result by reference, so the same object is shared across all validate() calls. Mutating one result then corrupts the default for every later call:

from schema import Schema, Optional
s = Schema({Optional("items", default=[]): list})
a = s.validate({}); a["items"].append(1)
b = s.validate({})
print(b["items"])   # [1]  -> should be []

Callable defaults (e.g. default=list) already returned a fresh object each time, which is why they don't have this bug.

Fix

Deep-copy non-callable defaults so each validation gets its own object:

new[default.key] = (
    _invoke_with_optional_kwargs(default.default, **kwargs)
    if callable(default.default)
    else copy.deepcopy(default.default)
)

Test

Added test_dict_optional_mutable_default_not_shared: mutating one result's default must not leak into a later validate(). It fails on master ([1]) and passes with this change. Full test_schema.py passes (121 tests).