The content-type header inserted into a multipart message part
is missing any params such as charset because its value is being
fetched via the swob.Response content_type property, which conforms
to webob spec and strips off all params.
This was noticed in work on feature/crypto branch because the crypto
meta param was being stripped off content-type in multipart messages,
preventing the content-type being decrypted. But in general there is
no suggestion in the multipart message spec [1] that params should
not be included. In fact examples in [1] show the charset param
included in the content-type value.
To ensure that the multipart message part content-type includes the
original content-type params, fetch it directly from the response
headers.
[1] http://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
Change-Id: Iff7274aa631a92cd7332212ed8b4378c27da4a1f
ad722b3ed3