mediatype: dedup parameters by exact name, not substring

[binggao1230]

fixMangledMediaType deduplicates media-type parameters (added for #162) with a substring test:

if strings.Contains(mtype, strings.TrimSpace(pair[0])) { // pair[0] == "name="
    continue
}

mtype is the parameter section accumulated so far, so a distinct parameter is dropped whenever its "<name>=" token appears in the earlier text — usually when the name is a suffix of an earlier one (name after filename, id after uuid, type after subtype), or when "<name>=" occurs inside an earlier value. Since filename and name co-occur in most attachment headers, application/octet-stream; filename="a"; name="b" loses name and the attachment filename is silently discarded.

These are all well-formed RFC 2045 headers that Go's mime.ParseMediaType returns intact, so it's a clean oracle for the intended result:

application/octet-stream; filename="a.txt"; name="b.txt"   -> name dropped
text/x; uuid="u"; id="7"                                    -> id dropped
text/plain; note="size=5"; size="10"                        -> size dropped

Fix: track seen parameter names in an exact, case-insensitive set (RFC 2045 §5.1 makes names case-insensitive) and skip only true duplicate keys. Exact repeats still collapse (the #162 behavior); distinct parameters are kept.

I diffed a broader well-formed battery (varied param order, quoted values containing ;/=, RFC 2231 extended params, encoded-words) against mime.ParseMediaType — the substring dedup was the only divergence; fixUnquotedSpecials, fixUnescapedQuotes and removeTrailingHTMLTags matched stdlib on every well-formed input. (#401 edits the same file but a different seam — RFC 2231 multi-segment reassembly — not this dedup.)

Added TestParseMediaType / TestFixMangledMediaType cases plus an end-to-end ReadParts test asserting the attachment FileName survives. go test ./... passes.

[coveralls]

coverage: 86.407% (+0.007%) from 86.4% — gaoflow:fix-mediatype-substring-param-drop into jhillyerd:main

[jhillyerd]

Nice, thanks