Make sure we clean preview_interceptors

backport https://github.com/rails/rails/pull/14318 to 4-1 .

I am 95% that this fixed the mailer issue on travis. 

travis is now failing randomly for another reason:
```
1) Error:
MemCacheStoreTest#test_deserializes_unloaded_class:
ArgumentError: key cannot be blank
```

So we should backport this to 4-1-stable. cc @pixeltrix