Match String#to_time's behaviour to ruby

Previously `String#to_time` returned the midnight of the current date
in some cases where there was no relavant information in the string.
Now the method returns `nil` instead in those cases.

Fixes #22958.

activesupport/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Match `String#to_time`'s behaviour to that of ruby's implementation for edge cases.
+
+    `nil` is now returned instead of the current date if the string provided does
+    contain time information, but none that is used to build the `Time` object.
+
+    Fixes #22958.
+
+    *Siim Liiser*
+
 *   Rely on the native DateTime#<=> implementation to handle non-datetime like
     objects instead of returning `nil` ourselves. This restores the ability
     of `DateTime` instances to be compared with a `Numeric` that represents an
@@ -57,6 +66,7 @@
 
     *Jon Moss*
 
+
 ## Rails 5.0.0.beta2 (February 01, 2016) ##
 
 *   Change `number_to_currency` behavior for checking negativity.
@@ -94,6 +104,7 @@
 
     *Akshay Vishnoi*
 
+
 ## Rails 5.0.0.beta1 (December 18, 2015) ##
 
 *   Add thread_m/cattr_accessor/reader/writer suite of methods for declaring class and module variables that live per-thread.

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

@@ -18,7 +18,8 @@ class String
   #   "12/13/2012".to_time               # => ArgumentError: argument out of range
   def to_time(form = :local)
     parts = Date._parse(self, false)
-    return if parts.empty?
+    used_keys = %i(year mon mday hour min sec sec_fraction offset)
+    return if (parts.keys & used_keys).empty?
 
     now = Time.now
     time = Time.new(

activesupport/test/core_ext/string_ext_test.rb

@@ -456,6 +456,7 @@ def test_string_to_time
       assert_equal Time.local(2011, 2, 27, 17, 50), "2011-02-27 13:50 -0100".to_time
       assert_equal Time.utc(2011, 2, 27, 23, 50), "2011-02-27 22:50 -0100".to_time(:utc)
       assert_equal Time.local(2005, 2, 27, 22, 50), "2005-02-27 14:50 -0500".to_time
+      assert_nil "010".to_time
       assert_nil "".to_time
     end
   end