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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