Fix safe_constantize to not raise a LoadError.

### Summary

There was an issues when using `safe_constantize` on a string that has
the wrong case.

File `em.rb` defines `EM`.
`"Em".safe_constantize` causes a little confusion with the autoloader.
The autoloader finds file "em.rb",
expecting it to define `Em`, but `Em` is not defined.
The autoloader raises a `LoadError`, which is good,
But `safe_constantize` is defined to return `nil` when a class is not found.

### Before

```
"Em".safe_constantize
LoadError: Unable to autoload constant Em, \
expected rails/activesupport/test/autoloading_fixtures/em.rb to define it
```

### After

```
"Em".safe_constantize
# => nil
```

2 files changed, 14 insertions, 0 deletions

diff --git a/activesupport/test/autoloading_fixtures/raises_load_error.rb b/activesupport/test/autoloading_fixtures/raises_load_error.rb
new file mode 100644
index 0000000000..f97be29b71
--- /dev/null
+++ b/activesupport/test/autoloading_fixtures/raises_load_error.rb
@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+# raises a load error typical of the dynamic code that manually raises load errors
+raise LoadError, "required gem not present kind of error"
diff --git a/activesupport/test/constantize_test_cases.rb b/activesupport/test/constantize_test_cases.rb
index 2c6145940b..cdb8441b81 100644
--- a/activesupport/test/constantize_test_cases.rb
+++ b/activesupport/test/constantize_test_cases.rb
@@ -112,6 +112,16 @@ module ConstantizeTestCases
     assert_nil yield("A::Object::B")
     assert_nil yield("A::Object::Object::Object::B")
 
+    with_autoloading_fixtures do
+      assert_nil yield("Em")
+    end
+
+    assert_raises(LoadError) do
+      with_autoloading_fixtures do
+        yield("RaisesLoadError")
+      end
+    end
+
     assert_raises(NameError) do
       with_autoloading_fixtures do
         yield("RaisesNameError")