Read the charset of the attachment if given in the raw MIME

[haggag]

The problem
The charset property isn't read upon parsing MIME like the following snippet:

Content-Type: text/plain; charset="US-ASCII"; name="attachment1.txt"
Content-Disposition: attachment; filename="attachment1.txt"
Content-Transfer-Encoding: base64
Content-ID: <f_jnt7un642>
X-Attachment-Id: f_jnt7un642

MQ==
--000000000000d551e4057998ef2b
Content-Type: text/plain; charset="US-ASCII"; name="attachement2.txt"
Content-Disposition: attachment; filename="attachement2.txt"
Content-Transfer-Encoding: base64
Content-ID: <f_jnt7un5p1>
X-Attachment-Id: f_jnt7un5p1

Mg==

Note that the charset="US-ASCII" is set for both attachments above!

but mail-parser doesn't parse charset. For example the following is the output of parsing the above attachments without charset:

[
    {
        "filename": "attachment1.txt",
        "payload": "MQ==",
        "binary": true,
        "mail_content_type": "text/plain",
        "content-id": "<f_jnt7un642>",
        "content_transfer_encoding": "base64"
    },
    {
        "filename": "attachement2.txt",
        "payload": "Mg==",
        "binary": true,
        "mail_content_type": "text/plain",
        "content-id": "<f_jnt7un5p1>",
        "content_transfer_encoding": "base64"
    }
]

The proposed solution
The parsed attachments objects should contain the 'charset' field if given in the MIME data.