Pull output buffer conditional up

This pulls the "output buffer existence" conditional up.  Instead of
evaling the same conditional over and over, we can pull it in to "only
compiled once" Ruby code.

actionview/lib/action_view/template.rb

@@ -155,7 +155,7 @@ def supports_streaming?
     # This method is instrumented as "!render_template.action_view". Notice that
     # we use a bang in this instrumentation because you don't want to
     # consume this in production. This is only slow if it's being listened to.
-    def render(view, locals, buffer = nil, &block)
+    def render(view, locals, buffer = ActionView::OutputBuffer.new, &block)
       instrument_render_template do
         compile!(view)
         view.send(method_name, locals, buffer, &block)

actionview/lib/action_view/template/handlers/erb/erubi.rb

@@ -13,7 +13,7 @@ def initialize(input, properties = {})
 
             # Dup properties so that we don't modify argument
             properties = Hash[properties]
-            properties[:preamble]   = "@output_buffer = output_buffer || ActionView::OutputBuffer.new;"
+            properties[:preamble]   = "@output_buffer = output_buffer;"
             properties[:postamble]  = "@output_buffer.to_s"
             properties[:bufvar]     = "@output_buffer"
             properties[:escapefunc] = ""
@@ -23,6 +23,8 @@ def initialize(input, properties = {})
 
           def evaluate(action_view_erb_handler_context)
             pr = eval("proc { #{@src} }", binding, @filename || "(erubi)")
+            # Double assignment to eliminate -w warnings
+            output_buffer = output_buffer = ActionView::OutputBuffer.new
             action_view_erb_handler_context.instance_eval(&pr)
           end