Fix DynamicStruct so that it allows a sort param.

`sort` params were broken due to the fact that we
are using a Hashie::Mash as our DynamicStruct. Hash
has a sort method, so it meant that a `sort` entry
in the hash was not used for `mash.sort`.

I don't remember the full history of why I switched to
a Hashie::Mash in 8f6d2f4 but this is one downside:
more collisions with method names. We may want to
reconsider that decision at some point, but this solves
the problem for now.

lib/interpol/dynamic_struct.rb

@@ -14,8 +14,12 @@ module DynamicStruct
       raise NoMethodError, "undefined method `#{key}' for #{hash.inspect}"
     end
 
+    Mash = Class.new(::Hashie::Mash) do
+      undef sort
+    end
+
     def self.new(source)
-      hash = Hashie::Mash.new(source)
+      hash = Mash.new(source)
       recursively_freeze(hash)
       hash
     end

spec/unit/interpol/dynamic_struct_spec.rb

@@ -33,6 +33,18 @@ module Interpol
       ds.a.should eq([1, 2, 3])
       ds.b.map(&:c).should eq([5, 4])
     end
+
+    hash_methods_allowed_as_params = [:sort]
+
+    hash_methods_allowed_as_params.each do |meth|
+      it 'allows params that correspond to hash method names' do
+        {}.should respond_to(meth)
+        ds = DynamicStruct.new(meth.to_s => "v1", "inner" => {
+          meth.to_s => "v2" })
+        ds.send(meth).should eq("v1")
+        ds.inner.send(meth).should eq("v2")
+      end
+    end
   end
 end