Add check to __contains__(k)

[zsandrus]

Fixes #16

Adds a check to see if the first level of the key exists before trying to retrieve the value.

[edwardselby]

not sure on the progress of this so i have made a work around for the time being, use at your own risk:

from dotted.collection import (
    DottedDict as BaseDottedDict,
    DottedList as BaseDottedList,
    DottedCollection as BaseDottedCollection
)


class DottedCollection(BaseDottedCollection):

    def __getitem__(self, name): return super().__getitem__(name)
    def __setitem__(self, name, value): super().__setitem__(name, value)
    def __delitem__(self, name): super().__delitem__(name)
    def to_python(self): return super().to_python()

    @classmethod
    def factory(cls, initial=None):
        if isinstance(initial, list):
            return DottedList(initial)
        elif isinstance(initial, dict):
            return DottedDict(initial)
        else:
            return initial

class DottedList(BaseDottedList, DottedCollection): pass

class DottedDict(BaseDottedDict, DottedCollection):
    def __contains__(self, k):
        try:
            return super().__contains__(k)
        except KeyError:
            return False

# proof of concept.
data = DottedCollection.factory({"calculated": {"attribute": 123}})
if "calculated.attribute" in data: print('test')

data = DottedCollection.factory([{"calculated": {"attribute": 123}}])
    for item in data:
        if "calculated.attribute" in data: print('test')

Please let us know when you fix this buddy.