Make it clear that SB#[]= takes 3 arguments, and reduce Array allocation

this reduces a redundant Array allocation that used to be created by *.
Plus, added some tests for []= with three arguments.

activesupport/lib/active_support/core_ext/string/output_safety.rb

@@ -88,11 +88,11 @@ def replace(value)
       super(implicit_html_escape_interpolated_argument(value))
     end
 
-    def []=(*args)
-      if args.length == 3
-        super(args[0], args[1], implicit_html_escape_interpolated_argument(args[2]))
+    def []=(arg1, arg2, arg3 = nil)
+      if arg3
+        super(arg1, arg2, implicit_html_escape_interpolated_argument(arg3))
       else
-        super(args[0], implicit_html_escape_interpolated_argument(args[1]))
+        super(arg1, implicit_html_escape_interpolated_argument(arg2))
       end
     end
 

activesupport/test/core_ext/string_ext_test.rb

@@ -1011,6 +1011,12 @@ def to_s
 
     assert_equal "<b>oo", string
     assert_predicate string, :html_safe?
+
+    string = "foo".html_safe
+    string[0, 2] = "<b>".html_safe
+
+    assert_equal "<b>o", string
+    assert_predicate string, :html_safe?
   end
 
   test "Replacing index of safe with unsafe yields escaped safe" do
@@ -1019,6 +1025,12 @@ def to_s
 
     assert_equal "&lt;b&gt;oo", string
     assert_predicate string, :html_safe?
+
+    string = "foo".html_safe
+    string[1, 1] = "<b>"
+
+    assert_equal "f&lt;b&gt;o", string
+    assert_predicate string, :html_safe?
   end
 
   test "emits normal string YAML" do