Fix `blob.service_url` for supports string or nil `:filename` option.

- Make sure `blob.service_url` present a `ActiveStorage::Filename` type to `serivce.url`.
- Add `ActiveStorage::Filename.wrap` method.

before:

```rb
blob.service_url(filename: ActiveStorage::Filename.new("new.txt"))

blob.service_url(filename: "new.txt")
=> NoMethodError: undefined method `parameters' for "new.txt":String

params = {}
blob.service_url(filename: params[:filename])
=> NoMethodError: undefined method `parameters' for nil:NilClass
```

after:

```rb
blob.service_url(filename: "new.txt")
blob.service_url(filename: nil)
```

1 files changed, 8 insertions, 0 deletions

diff --git a/activestorage/app/models/active_storage/filename.rb b/activestorage/app/models/active_storage/filename.rb
index b9413dec95..2b8880716e 100644
--- a/activestorage/app/models/active_storage/filename.rb
+++ b/activestorage/app/models/active_storage/filename.rb
@@ -5,6 +5,14 @@
 class ActiveStorage::Filename
   include Comparable
 
+  class << self
+    # Returns a Filename instance based on the given filename. If the filename is a Filename, it is
+    # returned unmodified. If it is a String, it is passed to ActiveStorage::Filename.new.
+    def wrap(filename)
+      filename.kind_of?(self) ? filename : new(filename)
+    end
+  end
+
   def initialize(filename)
     @filename = filename
   end