Replace (slower) block.call with (faster) yield

Performance optimization: `yield` with an implicit `block` is faster than `block.call`.
See http://youtu.be/fGFM_UrSp70?t=10m35s and the following benchmark:

```ruby
require 'benchmark/ips'

def fast
 yield
end

def slow(&block)
 block.call
end

Benchmark.ips do |x|
 x.report('fast') { fast{} }
 x.report('slow') { slow{} }
end

# => fast    154095 i/100ms
# => slow     71454 i/100ms
# =>
# => fast  7511067.8 (±5.0%) i/s -   37445085 in   4.999660s
# => slow  1227576.9 (±6.8%) i/s -    6145044 in   5.028356s
```

1 files changed, 2 insertions, 2 deletions

diff --git a/actionpack/lib/action_controller/metal/mime_responds.rb b/actionpack/lib/action_controller/metal/mime_responds.rb
index dc572f13d2..591f881a53 100644
--- a/actionpack/lib/action_controller/metal/mime_responds.rb
+++ b/actionpack/lib/action_controller/metal/mime_responds.rb
@@ -216,11 +216,11 @@ module ActionController #:nodoc:
     #
     # Be sure to check the documentation of +respond_with+ and
     # <tt>ActionController::MimeResponds.respond_to</tt> for more examples.
-    def respond_to(*mimes, &block)
+    def respond_to(*mimes)
       raise ArgumentError, "respond_to takes either types or a block, never both" if mimes.any? && block_given?
 
       collector = Collector.new(mimes, request.variant)
-      block.call(collector) if block_given?
+      yield collector if block_given?
 
       if format = collector.negotiate_format(request)
         _process_format(format)