mime: preserve unnecessary backslash escapes as literals
When MSIE sends a full file path (in "intranet mode"), it does not
escape backslashes: "C:\dev\go\foo.txt", not "C:\\dev\\go\\foo.txt".
No known MIME generators emit unnecessary backslash escapes
for simple token characters like numbers and letters.
If we see an unnecessary backslash escape, assume it is from MSIE
and intended as a literal backslash. This makes Go servers deal better
with MSIE without affecting the way they handle conforming MIME
generators.