Security: c.Attachment and c.Inline should escape filename in Content-Disposition header

[aldas]

This fixes #2531

c.Attachment and c.Inline should escape filename in Content-Disposition header to avoid 'Reflect File Download' vulnerability.

This is same as Go std does escaping https://github.com/golang/go/blob/9d836d41d0d9df3acabf7f9607d3b09188a9bfc6/src/mime/multipart/writer.go#L132

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (50ebcd8) 92.89% compared to head (1e63f47) 92.89%.

Additional details and impacted files

@@ Coverage Diff @@ ## master #2541 +/- ## ======================================= Coverage 92.89% 92.89% ======================================= Files 39 39 Lines 4658 4658 ======================================= Hits 4327 4327 Misses 240 240 Partials 91 91 

Files	Coverage Δ
context.go	88.88% <100.00%> (ø)

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