Use transform_values in a few more places

It's faster on Ruby 2.7+ since it avoids rehashing the keys.

activemodel/lib/active_model/errors.rb

@@ -317,12 +317,10 @@ def as_json(options = nil)
     #   person.errors.to_hash       # => {:name=>["cannot be nil"]}
     #   person.errors.to_hash(true) # => {:name=>["name cannot be nil"]}
     def to_hash(full_messages = false)
-      hash = {}
       message_method = full_messages ? :full_message : :message
-      group_by_attribute.each do |attribute, errors|
-        hash[attribute] = errors.map(&message_method)
+      group_by_attribute.transform_values do |errors|
+        errors.map(&message_method)
       end
-      hash
     end
 
     def to_h
@@ -348,9 +346,8 @@ def messages
     # Updating this hash would still update errors state for backward
     # compatibility, but this behavior is deprecated.
     def details
-      hash = {}
-      group_by_attribute.each do |attribute, errors|
-        hash[attribute] = errors.map(&:detail)
+      hash = group_by_attribute.transform_values do |errors|
+        errors.map(&:detail)
       end
       DeprecationHandlingDetailsHash.new(hash)
     end

activesupport/lib/active_support/json/decoding.rb

@@ -63,8 +63,8 @@ def convert_dates_from(data)
           when Array
             data.map! { |d| convert_dates_from(d) }
           when Hash
-            data.each do |key, value|
-              data[key] = convert_dates_from(value)
+            data.transform_values! do |value|
+              convert_dates_from(value)
             end
           else
             data