'Utility function that creates JSON repr. of a credentials object.
Over-ride is needed since PKCS#12 keys will not in general be JSON
serializable.
Args:
strip: array, An array of names of members to exclude from the
JSON.
to_serialize: dict, (Optional) The properties for this object
that will be serialized. This allows callers to
modify before serializing.
Returns:
string, a JSON representation of this instance, suitable to pass to
from_json().'

def _to_json(self, strip, to_serialize=None):

    if (to_serialize is None):
        to_serialize = copy.copy(self.__dict__)
    pkcs12_val = to_serialize.get(_PKCS12_KEY)
    if (pkcs12_val is not None):
        to_serialize[_PKCS12_KEY] = base64.b64encode(pkcs12_val)
    return super(ServiceAccountCredentials, self)._to_json(strip, to_serialize=to_serialize)
