Fix client.request mutable defaults causing invalid signatures

[khangah]

client.request is using mutable defaults for data and headers that affect the next request. If data is not replaced when the next client.request is called, it uses the previous request data, resulting in an invalid signature.

This code reproduces the issue:

client = Spot(api_key, api_secret)
print(client.account())
try:
    print(client.account()) # This function call will always fail with Invalid Signature
except Exception as e:
    print(e)