Supports checking String equality:

```ruby
filename = ActiveStorage::Filename.new("file.txt")
filename == "file.txt" # => true
filename in "file.txt" # => true
"file.txt" == filename # => true
```

Prior to this change, tests asserting equality would need to coerce the
`ActiveStorage::Filename` instance to a string:

```ruby
blob = # create ActiveStorage::Blob

assert_equal "file.txt", blob.filename.to_s
```

After this change, the equality check will automatically coerce:

```ruby
blob = # create ActiveStorage::Blob

assert_equal "file.txt", blob.filename
```

As an added benefit, the filename will match in `case` and `if ... in`
statements:

```ruby
blob = # create ActiveStorage::Blob
filename = blob.filename

case filename
when "file.txt" then ...
end

if filename in "file.txt"
  # ...
end
```