fix rails4 regression with asset_host Proc returning nil

In rails4, when config.asset_host is defined as a proc, returning nil from that proc no longer behaves the same as if config.asset_host were not configured, but rather treats asset_host as an empty string, which is not useful, because it gives urls like this:
```
  1) Failure:
AssetTagHelperTest#test_image_path_with_asset_host_proc_returning_nil [/Users/seanwalbran/projects/rails/actionpack/test/template/asset_tag_helper_test.rb:540]:
Expected: "/images/file.tiff"
  Actual: "http:///images/file.tiff"
```

This PR restores the previous behavior (and adds test coverage).