Sanitze HTTP header early and better

[drwetter]

On MacOS run_http_header() hiccuped when in any place of the web site unprintable chars were returned, see #2708 (comment) .

This PR fixes that by moving the sanitization to a separate function and run it earlier before any processing of the returned content (header plus body) takes place.

Output was:

'HTTP Status Code           awk: towc: multibyte conversion failure on: '� disabilitato");

 input record number 36, file /tmp/testssl.FHu8E0/AAA.BBB.CCC.DDD.http_header.txt
 source line number 1
'wk: towc: multibyte conversion failure on: '� disabilitato");

 input record number 36, file /tmp/testssl.FHu8E0/AAA.BBB.CCC.DDD.http_header.txt
 source line number 1
  200 OK

What is your pull request about?

• Bug fix
• Improvement
• New feature (adds functionality)
• Breaking change (bug fix, feature or improvement that would cause existing functionality to not work as expected)
• Typo fix
• Documentation update
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If it's a code change please check the boxes which are applicable

• For the main program: My edits contain no tabs, indentation is five spaces and any line endings do not contain any blank chars
• I've read CONTRIBUTING.md and Coding_Convention.md
• I have tested this fix or improvement against >=2 hosts and I couldn't spot a problem
• I have tested this new feature against >=2 hosts which show this feature and >=2 host which does not (in order to avoid side effects) . I couldn't spot a problem
• For the new feature I have made corresponding changes to the documentation and / or to help()
• If it's a bigger change: I added myself to CREDITS.md (alphabetical order) and the change to CHANGELOG.md