String#* takes one argument, and in that case, better not splat the arg

because splatting the argument allocates an extra Array object.

benchmark
```ruby
s = 'a'.html_safe
Benchmark.ips do |x|
  x.report('') { s * 1 }
end
```

result
```
before
Warming up --------------------------------------
                       216.816k i/100ms
Calculating -------------------------------------
                          2.341M (± 2.0%) i/s -     11.708M in   5.002555s

after
Warming up --------------------------------------
                       315.118k i/100ms
Calculating -------------------------------------
                          3.704M (± 1.5%) i/s -     18.592M in   5.020261s
```

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

@@ -100,7 +100,7 @@ def +(other)
       dup.concat(other)
     end
 
-    def *(*)
+    def *(_)
       new_string = super
       new_safe_buffer = new_string.is_a?(SafeBuffer) ? new_string : SafeBuffer.new(new_string)
       new_safe_buffer.instance_variable_set(:@html_safe, @html_safe)