This allows reformatting of inserted headers by canonicalizing tag case
and, crucially, folding (or refolding) lines.

When the header object is not explicitly marked modifiable — such as
when generated via parsing a raw MIME message — Mail::Header assumes
that header values you set should be inserted as-is.  This means
newlines are not stripped or validated as you're expected to construct
proper continuations yourself.

RT incorrectly assumed newlines in header values would be stripped,
leaving open the possibility of header injection via various
user-controlled inputs.  This commit resolves CVE-2012-4730.

Fixes failing tests by removing the assumptions that:

    1) Case of header names is preserved
    2) Header values are always on a single line