[CPP-QT-QHTTPENGINE Server] various bug fixes

[ipittau]

To close #11176

It's related to #10706

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QT Server was not building and not working because of three issues, fixed in this three commits.

@muttleyxd

[muttleyxd]

Tagging rest of C++ committee

@ravinikam @stkrwork @etherealjoy @MartinDelille

[muttleyxd]

I didn't check the regular expression, rest of the changes look fine.