do not use empty dict in parameter default

just fixed a bug in bitmask, along these lines:

# python gotchas 101

def foobar(headers={}, thing=False):
    if thing:
        print "WE MODIFY HEADERS"
        headers['if-modified'] = [thing]
    else:
        print "WE DO NOT MODIFY HEADERS"
    print headers

foobar(thing=42)
foobar(thing=23)
foobar(thing=False)

there are some occurrences in soledad:

/home/user/leap/soledad/tests/test_soledad/u1db_tests/test_http_client.py
[1] 304:                          self.getClient, creds={})

/home/user/leap/soledad/src/leap/soledad/common/couch/support.py
[2] 52:    def add(self, mimetype, content, headers={}):

/home/user/leap/soledad/src/leap/soledad/common/l2db/errors.py
[3] 146:    def __init__(self, status, message=None, headers={}):
[4] 163:    def __init__(self, message=None, headers={}):

/home/user/leap/soledad/src/leap/soledad/common/l2db/remote/http_app.py
[5] 456:    def start_response(self, status, obj_dic=None, headers={}):
[6] 476:    def send_response_json(self, status=200, headers={}, **kwargs):
[7] 481:    def send_response_content(self, content, status=200, headers={}):

they might be modifying the dict or not, but I think it's safer to use Nones.