joinTo/joinToString: Do not call toString on elements that are already CharSequence or Char.

#KT-15557

libraries/stdlib/test/collections/ArraysTest.kt

@@ -259,6 +259,22 @@ class ArraysTest {
     }
 
 
+    @Test fun joinToString() {
+        val text = arrayOf("foo", "bar").joinToString("-", "<", ">")
+        assertEquals("<foo-bar>", text)
+
+        val text2 = arrayOf('a', "b", StringBuilder("c"), null, "d", 'e', 'f').joinToString(limit = 4, truncated = "*")
+        assertEquals("a, b, c, null, *", text2)
+
+        val text3 = intArrayOf(1, 2, 5, 8).joinToString("+", "[", "]")
+        assertEquals("[1+2+5+8]", text3)
+
+        val text4 = charArrayOf('f', 'o', 'o').joinToString()
+        assertEquals("f, o, o", text4)
+    }
+
+
+
     @Test fun min() {
         expect(null, { arrayOf<Int>().min() })
         expect(1, { arrayOf(1).min() })