Use Hash#compact and Hash#compact! from Ruby 2.4

- Ruby 2.4 has added Hash#compact and Hash#compact! so we can use it
  now.
- Reference: https://bugs.ruby-lang.org/issues/11818 and https://bugs.ruby-lang.org/issues/12863.

activesupport/CHANGELOG.md

+*   Use `Hash#compact` and `Hash#compact!` from Ruby 2.4. Old Ruby versions
+    will continue to get these methods from Active Support as before.
+
+    *Prathamesh Sonpatki*
+
 *   Fix `ActiveSupport::TimeZone#strptime`.
     Support for timestamps in format of seconds (%s) and milliseconds (%Q).
 
@@ -21,10 +26,10 @@
         Time.zone = "US/Eastern"
 
         t = Time.zone.local(2016,11,6,1)
-        # => Sun, 06 Nov 2016 01:00:00 EDT -05:00 
+        # => Sun, 06 Nov 2016 01:00:00 EDT -05:00
 
         t.in(1.hour)
-        # => Sun, 06 Nov 2016 01:00:00 EST -05:00 
+        # => Sun, 06 Nov 2016 01:00:00 EST -05:00
 
     Fixes #26580.
 

activesupport/lib/active_support/core_ext/hash/compact.rb

 class Hash
-  # Returns a hash with non +nil+ values.
-  #
-  #   hash = { a: true, b: false, c: nil }
-  #   hash.compact        # => { a: true, b: false }
-  #   hash                # => { a: true, b: false, c: nil }
-  #   { c: nil }.compact  # => {}
-  #   { c: true }.compact # => { c: true }
-  def compact
-    select { |_, value| !value.nil? }
+  unless Hash.instance_methods(false).include?(:compact)
+    # Returns a hash with non +nil+ values.
+    #
+    #   hash = { a: true, b: false, c: nil }
+    #   hash.compact        # => { a: true, b: false }
+    #   hash                # => { a: true, b: false, c: nil }
+    #   { c: nil }.compact  # => {}
+    #   { c: true }.compact # => { c: true }
+    def compact
+      select { |_, value| !value.nil? }
+    end
   end
 
-  # Replaces current hash with non +nil+ values.
-  # Returns nil if no changes were made, otherwise returns the hash.
-  #
-  #   hash = { a: true, b: false, c: nil }
-  #   hash.compact!        # => { a: true, b: false }
-  #   hash                 # => { a: true, b: false }
-  #   { c: true }.compact! # => nil
-  def compact!
-    reject! { |_, value| value.nil? }
+  unless Hash.instance_methods(false).include?(:compact!)
+    # Replaces current hash with non +nil+ values.
+    # Returns nil if no changes were made, otherwise returns the hash.
+    #
+    #   hash = { a: true, b: false, c: nil }
+    #   hash.compact!        # => { a: true, b: false }
+    #   hash                 # => { a: true, b: false }
+    #   { c: true }.compact! # => nil
+    def compact!
+      reject! { |_, value| value.nil? }
+    end
   end
 end