[PATCH] Add Block Support to ActiveSupport OrderedHash merge

ActiveSupport's OrderedHash's implementation of merge and merge! does not support passing a block as per the standard library (c.f. http://ruby-doc.org/core/classes/Hash.html#M002880 )

The attached patch adds support to match the standard library's method signature:

hsh.merge(other_hash){|key, oldval, newval| block} → a_hash

Tests are also included.

This makes it possible to effectively map over an OrderedHash without turning it into an array by merging it with itself:

a = ActiveSupport::OrderedHash.new
a["age"] = 1
a["height"] = 15
a.merge(a) { |k, v| v * 2 }
# => #<OrderedHash {"height"=>30, "age"=>2}>