Replace simple callback functions with arrow functions

[Snuffleupagus]

This improves consistency in the code-base, and it's also slightly shorter which cannot hurt.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.28%. Comparing base (74515c6) to head (cd8c463).

Files with missing lines	Patch %	Lines
src/display/content_disposition.js	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21879   +/-   ##
=======================================
  Coverage   90.27%   90.28%           
=======================================
  Files         264      264           
  Lines       67275    67274    -1     
=======================================
+ Hits        60734    60739    +5     
+ Misses       6541     6535    -6     

Flag	Coverage Δ
browsertest	66.31% <0.00%> (+<0.01%)	⬆️
fonttest	8.92% <ø> (ø)
integrationtest	69.44% <0.00%> (+0.01%)	⬆️
unittest	58.48% <33.33%> (+<0.01%)	⬆️
unittestcli	57.02% <33.33%> (+<0.01%)	⬆️

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[Snuffleupagus]

/botio browsertest

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.241.84.105:8877/209142488906376/output.txt

[moz-tools-bot]

From: Bot.io (Windows)

---

Received

Command cmd_browsertest from @Snuffleupagus received. Current queue size: 0

Live output at: http://54.193.163.58:8877/da392b07ebf95af/output.txt

[moz-tools-bot]

From: Bot.io (Linux m4)

---

Success

Full output at http://54.241.84.105:8877/209142488906376/output.txt

Total script time: 19.03 mins

• Regression tests: Passed

[moz-tools-bot]

From: Bot.io (Windows)

---

Success

Full output at http://54.193.163.58:8877/da392b07ebf95af/output.txt

Total script time: 23.93 mins

• Regression tests: Passed

[timvandermeij]

This seems more readable indeed; thanks!