Fix printing of extension methods

Author: odersky

compiler/src/dotty/tools/dotc/printing/RefinedPrinter.scala

@@ -709,7 +709,7 @@ class RefinedPrinter(_ctx: Context) extends PlainPrinter(_ctx) {
     val (leading, paramss) =
       if (isExtension && vparamss.nonEmpty) (paramsText(vparamss.head) ~ " " ~ txt, vparamss.tail)
       else (txt, vparamss)
-    (txt /: paramss)((txt, params) =>
+    (leading /: paramss)((txt, params) =>
       txt ~
       (Str(" given ") provided params.nonEmpty && params.head.mods.is(Given)) ~
       paramsText(params))