Validate content type header parameter

[qmuntal]

RFC 9052 Section 3.1 impose this restrictions on the content type textual format:

Text values follow the syntax of "/", where and are defined in Section 4.2 of [RFC6838]. Leading and trailing whitespace is not permitted.

We are currently not performing any of those validations. This PR fix that.

[codecov[bot]]

Codecov Report

Merging #176 (5508600) into main (e7ac36d) will increase coverage by 0.20%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   91.99%   92.19%   +0.20%     
==========================================
  Files          12       12              
  Lines        1961     1973      +12     
==========================================
+ Hits         1804     1819      +15     
+ Misses        108      105       -3     
  Partials       49       49              

Files	Coverage Δ
headers.go	93.64% <100.00%> (+0.94%)	:arrow_up:

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[SteveLasker]

Awaiting @qmuntal to resolve the merge conflict, then we can merge.