Fix transaction config query generation

Closes rust-postgres#187

Author: sfackler

src/transaction.rs

@@ -88,7 +88,7 @@ impl ConfigInternals for Config {
         }
 
         if let Some(read_only) = self.read_only {
-            if first {
+            if !first {
                 s.push(',');
             }
             if read_only {
@@ -100,7 +100,7 @@ impl ConfigInternals for Config {
         }
 
         if let Some(deferrable) = self.deferrable {
-            if first {
+            if !first {
                 s.push(',');
             }
             if deferrable {

tests/test.rs

@@ -1039,6 +1039,13 @@ fn transaction_config() {
     conn.set_transaction_config(&config).unwrap();
 }
 
+#[test]
+fn transaction_config_one_setting() {
+    let conn = Connection::connect("postgres://postgres@localhost", SslMode::None).unwrap();
+    conn.set_transaction_config(&transaction::Config::new().read_only(true)).unwrap();
+    conn.set_transaction_config(&transaction::Config::new().deferrable(true)).unwrap();
+}
+
 #[test]
 fn transaction_with() {
     let conn = Connection::connect("postgres://postgres@localhost", SslMode::None).unwrap();