jsonrpc2 requests need mandatory key/pair jsonrpc:'2.0'

closes 1898
jsonrpc2 errors out with error number -32600 if the request dict does not have a key 'jsonrpc' with value '2.0'
There was no mechanism for the ServerProxy class to know if it was sending a request to jsonrpc2 or jsonrpc so I added a keyword argument version=None
If defined, it adds the missing key to data. Only a value of version='2.0' will work with jsonrpc2.
None falls back to current jsonrpc behaviour.