Client fails if auth type is x509_proxy and X509_USER_PROXY is not set
The client fails if the authentication type is x509_proxy and the X509_USER_PROXY environment variable is not set with a very confusing error message:
```
$ rucio whoami
2018-09-27 09:22:57,749	ERROR	coercing to Unicode: need string or buffer, module found
```

diff --git a/lib/rucio/client/baseclient.py b/lib/rucio/client/baseclient.py
index 1174c41b4..1562a75c6 100644
--- a/lib/rucio/client/baseclient.py
+++ b/lib/rucio/client/baseclient.py
@@ -182,7 +182,7 @@ class BaseClient(object):
                         else:
                             fname = '/tmp/x509up_u%d' % geteuid()
                             if path.exists(fname):
-                                self.creds['client_proxy'] = path
+                                self.creds['client_proxy'] = fname
                             else:
                                 raise MissingClientParameter('Cannot find a valid X509 proxy; not in %s, $X509_USER_PROXY not set, and '
                                                              '\\'x509_proxy\\' not set in the configuration file.' % fname)
['lib/rucio/client/baseclient.py']