fix: never use credentials in combination with plain text (#98)

Author: olavloite

google-cloud-spanner/src/main/java/com/google/cloud/spanner/SpannerOptions.java

@@ -20,6 +20,7 @@
 import com.google.api.gax.grpc.GrpcInterceptorProvider;
 import com.google.api.gax.retrying.RetrySettings;
 import com.google.api.gax.rpc.TransportChannelProvider;
+import com.google.cloud.NoCredentials;
 import com.google.cloud.ServiceDefaults;
 import com.google.cloud.ServiceOptions;
 import com.google.cloud.ServiceRpc;
@@ -518,6 +519,8 @@ public ManagedChannelBuilder apply(ManagedChannelBuilder builder) {
                 return builder.usePlaintext();
               }
             });
+        // As we are using plain text, we should never send any credentials.
+        this.setCredentials(NoCredentials.getInstance());
       }
       return new SpannerOptions(this);
     }