smsg: use capitalization for header retrieval

smsg: use capitalization for header retrieval

PublicInbox::Eml will have case-sensitive memoization to
avoid the need to call `lc' to retrieve common headers,
so ensure we call $mime->header() with the common
capitalization.

Unfortunately, we need to continue using lowercase for field
names for smsg, since NNTP requires case-insensitivity when
matching headers and method dispatch is expensive.