imapserver: for the "bodystructure" fetch response item, add the content-type parameters for multiparts so clients will get the mime boundary without having to parse the message themselves

"bodystructure" is like "body", but bodystructure allows returning more
information. we chose not to do that, initially because it was easier to
implement, and more recently because we can't easily return the additional
content-md5 field for leaf parts (since we don't have it in parsed form). but
now we just return the extended form for multiparts, and non-extended form for
leaf parts. likely no one would be looking for any content-md5-value for leaf
parts anyway. knowing the boundary is much more likely to be useful.

for issue #217 by danieleggert, thanks for reporting!

imapserver/fetch_test.go

@@ -241,6 +241,7 @@ func TestFetch(t *testing.T) {
 						imapclient.BodyTypeBasic{MediaType: "IMAGE", MediaSubtype: "JPEG", BodyFields: imapclient.BodyFields{CTE: "BASE64"}},
 					},
 					MediaSubtype: "PARALLEL",
+					Ext:          &imapclient.BodyExtensionMpart{Params: [][2]string{{"boundary", "unique-boundary-2"}}},
 				},
 				imapclient.BodyTypeText{MediaType: "TEXT", MediaSubtype: "ENRICHED", BodyFields: imapclient.BodyFields{Octets: 145}, Lines: 5},
 				imapclient.BodyTypeMsg{
@@ -260,6 +261,7 @@ func TestFetch(t *testing.T) {
 				},
 			},
 			MediaSubtype: "MIXED",
+			Ext:          &imapclient.BodyExtensionMpart{Params: [][2]string{{"boundary", "unique-boundary-1"}}},
 		},
 	}
 	tc.client.Append("inbox", nil, &received, []byte(nestedMessage))