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
```

1 files changed, 4 insertions, 0 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 360cef2b41..d4eaee9f6d 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,7 @@
+*   Fix `String#safe_constantize` throwing a `LoadError` for incorrectly cased constant references.
+
+    *Keenan Brock*
+
 *   Preserve key order passed to `ActiveSupport::CacheStore#fetch_multi`.
 
     `fetch_multi(*names)` now returns its results in the same order as the `*names` requested, rather than returning cache hits followed by cache misses.