drop array allocations on `html_safe`

For better or worse, anonymous `*` args will allocate arrays.  Ideally,
the interpreter would optimize away this allocation.  However, given the
number of times we call `html_safe` it seems worth the shedding idealism
and going for performance.  This line was the top allocation spot for a
scaffold (and presumably worse on real applications).

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

@@ -171,7 +171,7 @@ def safe_concat(value)
       original_concat(value)
     end
 
-    def initialize(*)
+    def initialize(str = '')
       @html_safe = true
       super
     end