Default to using username when database isn't present on startup (#234)

src/client.rs

@@ -359,17 +359,17 @@ where
         let stats = get_reporter();
         let parameters = parse_startup(bytes.clone())?;
 
-        // These two parameters are mandatory by the protocol.
-        let pool_name = match parameters.get("database") {
-            Some(db) => db,
-            None => return Err(Error::ClientError),
-        };
-
+        // This parameter is mandatory by the protocol.
         let username = match parameters.get("user") {
             Some(user) => user,
             None => return Err(Error::ClientError),
         };
 
+        let pool_name = match parameters.get("database") {
+            Some(db) => db,
+            None => username,
+        };
+
         let application_name = match parameters.get("application_name") {
             Some(application_name) => application_name,
             None => "pgcat",