Fix malformed content-type header causing exception on charset get (v2.x)

[ruimarinho]

Ported #897 to v2.x branch as requested.

[codecov]

Codecov Report

Merging #898 into v2.x will not change coverage.
The diff coverage is 100%.

@@         Coverage Diff         @@
##           v2.x   #898   +/-   ##
===================================
  Coverage   100%   100%           
===================================
  Files         4      4           
  Lines       418    421    +3     
  Branches    100    100           
===================================
+ Hits        418    421    +3

Impacted Files	Coverage Δ
lib/request.js	100% <100%> (ø)	✅

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