Fix BTCChina signature method for string and boolean parameters.

BTCChina API is now using non-standard JSON serialization method to serialize the message for signature.
The quotes of string are stripped, boolean true is serialized to 1, and false is empty.

See the note of 4.2 USING OPENONLY AS TRUE EXAMPLE in [FAQ](http://btcchina.org/api-trade-documentation-en#faq):
> For hash generation, if the parameter is set as true, use 1, and for false as empty (“”). This will change in the future to follow JSON RPC standard for hash generation, and you will be alerted for this change.
I was considering to add a method `String getSignParams()` in `BTCChinaRequest` to return the params for signing, and refactor the `BTCChinaDigest.digestParams`. But seems there is no way(`restInvocation.getUnannanotatedParams()`?) to get the instance of `BTCChinaRequest` from `BTCChinaDigest.digestParams(RestInvocation)`.