Issues with handling quotation marks in attachment names

I came across to this issue in Ruby On Rails (https://github.com/rails/rails/pull/14158), and have traced it, and it seems that it might be actually a bug in `mail`-gem.

I have tried to fix it, but my dirty fix makes one other test to fail:

``` shell
1) Mail::UnstructuredField displaying encoded field and decoded value should encode additional special characters inside encoded-word-encoded strings
Failure/Error: @field.encoded.should eq result

expected: "Subject: =?UTF-8?Q?Her_er_=C3=A6=28=29<>@,;:\\=22/[]=3F.=3D?=\r\n"
got: "Subject: =?UTF-8?Q?Her_er_=C3=A6=28=29<>@,;:=22/[]=3F.=3D?=\r\n"

(compared using ==)

Diff:
@@ -1,2 +1,2 @@
-Subject: =?UTF-8?Q?Her_er_=C3=A6=28=29<>@,;:\=22/[]=3F.=3D?=
+Subject: =?UTF-8?Q?Her_er_=C3=A6=28=29<>@,;:=22/[]=3F.=3D?=
```

My "fix": https://github.com/jamox/mail/blob/fixing_fail_when_name_has_quotes/lib/mail/encodings.rb#L98-L110

---

Error occurs when in: https://github.com/mikel/mail/blob/master/lib/mail/attachments_list.rb#L40

``` ruby
def []=(name, value)
encoded_name = Mail::Encodings.decode_encode name, :encode
default_values = { :content_type => "#{set_mime_type(name)}; filename=\"#{encoded_name}\"",
:content_transfer_encoding => "#{guess_encoding}",
:content_disposition => "#{@content_disposition_type}; filename=\"#{encoded_name}\"" }
```

we have `name = 'invoice "for you".pdf'` and then we insert it into

``` ruby
default_values = { :content_type => "#{set_mime_type(name)}; filename=\"#{encoded_name}\"",
```

Thus getting

``` ruby
default_values = { :content_type => "#{set_mime_type(name)}; filename=\"invoice \"for you\".pdf\"",
```

And later we get the filename `\"invoice \"for you\".pdf\"` => `"invoice "for you".pdf"`...

Id be happy to fix this (in case this ain't due to misuse of this library) :)

Start with lib/mail/attachments_list.rb around the []= method and lib/mail/encodings.rb around the referenced encoding logic; run the failing Mail::UnstructuredField test shown in the report. The work is done when attachment names containing quotation marks are handled correctly and the existing encoding test passes without the reported regression.

Written by the indexing model from the issue text.