Missed optimizations

- freezing constant string to ensure it's not mutated
- use constant where available
- optimize `respond_to?` to not allocate array.

Discussed in #737 and #739

`respond_to?` takes two arguments all recent rubies:

- http://ruby-doc.org/core-2.1.3/Object.html#method-i-respond_to-3F
- http://ruby-doc.org/core-1.9.3/Object.html#method-i-respond_to-3F
- http://ruby-doc.org/core-1.8.7/Object.html#method-i-respond_to-3F

Also `method_missing` will return a symbol from the first argument, we don't need to allocate a string:

- http://ruby-doc.org/core-2.1.3/BasicObject.html#method-i-method_missing
- http://ruby-doc.org/core-1.9.3/BasicObject.html#method-i-method_missing
- http://ruby-doc.org/core-1.8.7/Kernel.html#method-i-method_missing

cc/ @raggi 
Isn't the code provided by @raggi https://github.com/rack/rack/pull/739#issuecomment-57698025 better?.