Implement fetch_values for HashWithIndifferentAccess (#28316)

`fetch_values` was added to Hash in Ruby 2.3.0:
  https://bugs.ruby-lang.org/issues/10017

This patch adds an implemention for instances of HWAI, in line
with the existing definitions of `fetch` and `values_at`.

activesupport/CHANGELOG.md

@@ -1 +1,7 @@
+*   Add `fetch_values` for `HashWithIndifferentAccess`
+
+    The method was originally added to `Hash` in Ruby 2.3.0.
+
+    *Josh Pencheon*
+
 Please check [5-1-stable](https://github.com/rails/rails/blob/5-1-stable/activesupport/CHANGELOG.md) for previous changes.

activesupport/lib/active_support/hash_with_indifferent_access.rb

@@ -195,6 +195,19 @@ module ActiveSupport
       indices.collect { |key| self[convert_key(key)] }
     end
 
+    # Returns an array of the values at the specified indices, but also
+    # raises an exception when one of the keys can't be found.
+    #
+    #   hash = ActiveSupport::HashWithIndifferentAccess.new
+    #   hash[:a] = 'x'
+    #   hash[:b] = 'y'
+    #   hash.fetch_values('a', 'b') # => ["x", "y"]
+    #   hash.fetch_values('a', 'c') { |key| 'z' } # => ["x", "z"]
+    #   hash.fetch_values('a', 'c') # => KeyError: key not found: "c"
+    def fetch_values(*indices, &block)
+      indices.collect { |key| fetch(key, &block) }
+    end if Hash.method_defined?(:fetch_values)
+
     # Returns a shallow copy of the hash.
     #
     #   hash = ActiveSupport::HashWithIndifferentAccess.new({ a: { b: 'b' } })

activesupport/test/hash_with_indifferent_access_test.rb

@@ -166,6 +166,18 @@ class HashWithIndifferentAccessTest < ActiveSupport::TestCase
     assert_equal [1, 2], @mixed.values_at(:a, :b)
   end
 
+  def test_indifferent_fetch_values
+    skip unless Hash.method_defined?(:fetch_values)
+
+    @mixed = @mixed.with_indifferent_access
+
+    assert_equal [1, 2], @mixed.fetch_values("a", "b")
+    assert_equal [1, 2], @mixed.fetch_values(:a, :b)
+    assert_equal [1, 2], @mixed.fetch_values(:a, "b")
+    assert_equal [1, "c"], @mixed.fetch_values(:a, :c) { |key| key }
+    assert_raise(KeyError) { @mixed.fetch_values(:a, :c) }
+  end
+
   def test_indifferent_reading
     hash = HashWithIndifferentAccess.new
     hash["a"] = 1