Use DateTime.parse inside String#to_datetime

Use the standard library's `DateTime.parse` because it's marginally
faster and supports partial date/time strings.

Benchmark:
       user     system      total        real
old  3.980000   0.000000   3.980000 (  3.987606)
new  3.640000   0.010000   3.650000 (  3.641342)

activesupport/lib/active_support/core_ext/string/conversions.rb

@@ -52,13 +52,6 @@ def to_date
   #   "2012-12-13 12:50".to_datetime    #=> Thu, 13 Dec 2012 12:50:00 +0000
   #   "12/13/2012".to_datetime          #=> ArgumentError: invalid date
   def to_datetime
-    unless blank?
-      date_values = ::Date._parse(self, false).
-        values_at(:year, :mon, :mday, :hour, :min, :sec, :zone, :sec_fraction).
-        map! { |arg| arg || 0 }
-      date_values[5] += date_values.pop
-
-      ::DateTime.civil(*date_values)
-    end
+    ::DateTime.parse(self, false) unless blank?
   end
 end

activesupport/test/core_ext/string_ext_test.rb

@@ -327,6 +327,12 @@ def test_string_to_datetime
     assert_nil "".to_datetime
   end
 
+  def test_partial_string_to_datetime
+    now = DateTime.now
+    assert_equal DateTime.civil(now.year, now.month, now.day, 23, 50), "23:50".to_datetime
+    assert_equal DateTime.civil(now.year, now.month, now.day, 23, 50, 0, "-04:00"), "23:50 -0400".to_datetime
+  end
+
   def test_string_to_date
     assert_equal Date.new(2005, 2, 27), "2005-02-27".to_date
     assert_nil "".to_date