Do not assume that bitcoin.conf file specifies rpcuser and rpcpassword

You can make RPC calls to bitcoind when bitcoin.conf specifies e.g.
just rpcpassword and leaves user name empty.  The class bitcoin.rpc.RawProxy
creates keys of the internal dictionary: conf = {}, based on the entries
in the configuration file.  Then  assumes that both rpcuser and rpcpassword
keys exist and uses them to create service_url.  This results in an error
when e.g. bitcoin.conf contains only rcppassword field and the following code
is executed:
>    proxy = bitcoin.rpc.RawProxy()
Simple fix: create the dictionary that already contains necessary keys.