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 might oversee things, but perhaps an easier solution is to encode that logic in `BTCChinaGetOrdersRequest` by generating the correct params string depending on the constructor input? Adding a new library for a single function seems heavy.