crawler: fix empty enctype case

[devl00p]

Saw since 85097cb#diff-d7e9c3756fcd51d905c37cb6d0f0ce60e24934ff30e015bde1b6a354d7e11edcR339 that requests may be sent with the "content-type" header specifying an empty value.

The MR fix that and also removes async_send that is directly handled by async_request now.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.41%. Comparing base (f6747b4) to head (459168c).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #614      +/-   ##
==========================================
+ Coverage   80.39%   80.41%   +0.01%     
==========================================
  Files         127      127              
  Lines       11408    11380      -28     
==========================================
- Hits         9172     9151      -21     
+ Misses       2236     2229       -7     

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[devl00p]

error could be seend in that CI: https://github.com/wapiti-scanner/wapiti/actions/runs/10593274361/job/29354416515

         "http_request": "ABC /admin/private.php HTTP/1.1\nhost: htaccess\nconnection: keep-alive\nuser-agent: Mozilla/5.0 (Windows NT 6.1; rv:45.0) Gecko/20100101 Firefox/45.0\naccept-language: en-US\naccept-encoding: gzip, deflate, br\naccept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8\ncontent-type:",