Fix charp JSONProtocol.ReadJSONDouble (specify InvariantCulture)

Hello, guys.

I caught error on serialization from application working under localized (Russian) Windows. Cause Double.Parse wait a system decimal separator. I fixed JSONProtocol.ReadJSONDouble method, made Double.Parse call with explicit specify InvariantCulture in second parameter.

I think my little fix is pretty clear for you. But if it's not, I would be glad to explain more details.

Sincerely, Alexander Makarov.