Better detection is_logged_in

[RMI78]

Answer to #317 @tarraschk
TL;DR: This commit raise a false flag as soon as it detects error messages from the response, and unit tests have been added.

This is only a small improvement, but we can only go little by little to improve this feature, since knowing if we are logged in regardless of which website we are on is a thought challenge.

Unless someone has a genuine idea, I leave here things that already have been explored:

• Checking redirections by replaying the request on the login page: may not work because some websites doesn't redirect when you are logged in, and doesn't redirect either when you fail to log in
• Checking cookies state before and after the attempt of login: may work but should be implemented cautiously by exploring a large amount of cases (some websites uses cookies for the sessions, some others don't + we should differentiate other cookies from the logging ones)

A good approach would be a smart combination of all those checks with a prioritizing system, but it might be a better idea to move this mechanism somewhere else than in the parser because a simple method in the html_parser wouldn't be enough. A full object might be required in the future to take advantages of all the information we can have on a higher scope

Again, we should be very cautious when improving this method since we want to minimize false negative but absolutely want to avoid false positive.

[codecov-commenter]

Codecov Report

Merging #473 (83193c5) into master (3acba90) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master     #473   +/-   ##
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  Coverage   77.70%   77.71%           
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  Files         104      104           
  Lines        8786     8789    +3     
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+ Hits         6827     6830    +3     
  Misses       1959     1959           

Files Changed	Coverage Δ
wapitiCore/parsers/html_parser.py	92.59% <100.00%> (+0.06%)	⬆️