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, 2 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 1af9833d46..ee193add6f 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -328,6 +328,8 @@ module ActiveSupport
         e.name.to_s == camel_cased_word.to_s)
     rescue ArgumentError => e
       raise unless /not missing constant #{const_regexp(camel_cased_word)}!$/.match?(e.message)
+    rescue LoadError => e
+      raise unless /Unable to autoload constant #{const_regexp(camel_cased_word)}/.match?(e.message)
     end
 
     # Returns the suffix that should be added to a number to denote the position