JS: fix char boxing in elvis expressions (#KT-17700 fixed)

js/js.translator/testData/box/char/charElvis.kt

@@ -0,0 +1,14 @@
+package foo
+
+fun box(): String {
+
+    assertEquals("1", "" + ('1' ?: return "fail1"))
+    assertEquals("2", "" + (null ?: '2'))
+    val c3: Char? = '3'
+    assertEquals("3", "" + (c3 ?: return "fail3"))
+
+    val c4: Char? = null
+    assertEquals("4", "" + (c4 ?: "4"))
+
+    return "OK"
+}