Fixed copying of aggregates in JS.

tests/js/tcopying.nim

@@ -0,0 +1,13 @@
+discard """
+  output: '''123
+'''
+"""
+
+type MyArray = array[1, int]
+
+proc changeArray(a: var MyArray) =
+    a = [123]
+
+var a : MyArray
+changeArray(a)
+echo a[0]