JS: fix copying fun with default args from interface to abstract class

See KT-20451

compiler/testData/codegen/box/defaultArguments/inheritedFromInterfaceViaAbstractSuperclass.kt

@@ -0,0 +1,17 @@
+interface I {
+    fun foo(x: Int = 23): String
+}
+
+abstract class Base : I
+
+class C : Base(), I {
+    override fun foo(x: Int) = "C:$x"
+}
+
+fun box(): String {
+    val x: I = C()
+    val r = x.foo() + ";" + x.foo(42)
+    if (r != "C:23;C:42") return "fail: $r"
+
+    return "OK"
+}