replace usage of `returning` with `each_with_object`

`each_with_object` is a Ruby 1.9 method, here re-implemented in core_ext.rb
in case it's missing (for older Ruby versions). Using `returning` is bad in
combination with Ruby on Rails because each usage of the method will emit
a Rails deprecation warning. This might be considered an Active Support bug,
but it's better to avoid using `returning` altogether and use `tap` from
Ruby 1.8.7, also re-implemented here in case it's missing.

Since existing usages or `returning` were better suited for `each_with_object`
than `tap`, they were rewritten using the former instead.

lib/mongo/connection.rb

@@ -277,8 +277,8 @@ module Mongo
     # @return [Hash]
     def database_info
       doc = self['admin'].command({:listDatabases => 1})
-      returning({}) do |info|
-        doc['databases'].each { |db| info[db['name']] = db['sizeOnDisk'].to_i }
+      doc['databases'].each_with_object({}) do |db, info|
+        info[db['name']] = db['sizeOnDisk'].to_i
       end
     end
 

lib/mongo/cursor.rb

@@ -321,9 +321,7 @@ module Mongo
           {fields => 1}
         when Array
           return nil if fields.length.zero?
-          returning({}) do |hash|
-            fields.each { |field| hash[field] = 1 }
-          end
+          fields.each_with_object({}) { |field, hash| hash[field] = 1 }
         when Hash
           return fields
       end

lib/mongo/util/core_ext.rb

@@ -20,10 +20,21 @@
 class Object
 
   #:nodoc:
-  def returning(value)
-    yield value
-    value
-  end
+  def tap
+    yield self
+    self
+  end unless respond_to? :tap
+
+end
+
+#:nodoc:
+module Enumerable
+
+  #:nodoc:
+  def each_with_object(memo)
+    each { |element| yield(element, memo) }
+    memo
+  end unless [].respond_to?(:each_with_object)
 
 end