fix regular expression

The original regular expression will match the string like `inline;filename=""`, and `match[2]` returns `""`.

I found this issue when I did a remote image download from google+ avatar.
For example: https://lh3.googleusercontent.com/-Y86IN-vEObo/AAAAAAAAAAI/AAAAAAACk4w/yvxY4GMx_8k/photo.jpg

I expect the `original_filename` should be `photo.jpg`, but it always returns `__` due to its `content-disposition` is `inline;filename=""`.

After read [RFC2616](http://www.w3.org/Protocols/rfc2616/rfc2616-sec19.html).

> filename-parm = "filename" "=" quoted-string

It seems quoted string is required. I suggest replace `/filename=(\"?)(.+)\1/` by `/filename=(\")(.+)\1/` so that `match.nil?` will return true, and `original_filename` will fallback to `"photo.jpg"`.