fix(middleware): do not add 'null' to Content-Type

[clshortfuse]

fixes: #354

What kind of change does this PR introduce?

bugfix

Did you add tests for your changes?

Of course!

Summary

Do not pass 'null' as media-type when mime-type lookup fails.

See #354

Does this PR introduce a breaking change?

No

[jsf-clabot]

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[codecov]

Codecov Report

Merging #355 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #355   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files           7        7           
  Lines         252      252           
=======================================
  Hits          244      244           
  Misses          8        8

Impacted Files	Coverage Δ
lib/middleware.js	94.33% <100%> (ø)	⬆️

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[codecov]

Codecov Report

Merging #355 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #355   +/-   ##
=======================================
  Coverage   96.82%   96.82%           
=======================================
  Files           7        7           
  Lines         252      252           
=======================================
  Hits          244      244           
  Misses          8        8

Impacted Files	Coverage Δ
lib/middleware.js	94.33% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e6cfe7c...dc9aa4e. Read the comment docs.

[alexander-akait]

Good job, thanks!