Fix proxy overwrite bug in rpc-client

When there are multiple records in the same namespace, rpc-client will overwrite proxy with the latter record.
 For example:
records = [ { namespace: 'user', serverType: 'test', path: './test1'}, {namespace: 'user', serverType: 'test', path: './test2'}];
Then test2's proxy will overwrite test1's proxy.