`assert_changes` should always assert some change

While using `assert_changes`, I came across some unexpected behavior:
if you provide a `to:` argument, and the expression matches but didn't
actually change, the assertion will pass.

The way `assert_changes` reads, I assumed that it would both assert
that there was any change at all, _and_ that the expression changed to
match my `to:` argument.

In the case of just a `from:` argument, `assert_changes` does what I
expect as well. It asserts that the before value `=== from` and that
the after value changed.

My key change is that `assert_changes` will now _always_ assert that
expression changes, no matter what combination of `from:` and `to:`
arguments

1 files changed, 6 insertions, 5 deletions

diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index b24aa36ede..6f69c48674 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/activesupport/lib/active_support/testing/assertions.rb
@@ -156,11 +156,12 @@ module ActiveSupport
 
         after = exp.call
 
-        if to == UNTRACKED
-          error = "#{expression.inspect} didn't change"
-          error = "#{message}.\n#{error}" if message
-          assert before != after, error
-        else
+        error = "#{expression.inspect} didn't change"
+        error = "#{error}. It was already #{to}" if before == to
+        error = "#{message}.\n#{error}" if message
+        assert before != after, error
+
+        unless to == UNTRACKED
           error = "#{expression.inspect} didn't change to #{to}"
           error = "#{message}.\n#{error}" if message
           assert to === after, error