Basket id string

As per https://github.com/7digital/SevenDigital.Api.Wrapper/issues/52
- basket ids are stings.
- setting a basket id no longer clears other parameters. If you want to clear params, make a new request or call ClearParameters() . This a appears to be a workaround for threading/concurrency issues.
I don't know if I understand this right, if there's some complication that i missed then don't merge.