Fix type hints and add undefined property in Webapi [2.3-develop]

[mhauri]

Description

• Fix type hints in Webapi/Controller/Soap/Request/Handler.php and Webapi/Model/Plugin/GuestAuthorization.php

• Fix incorrect case in property annotation in Soap\Server.php on line 34

• add undefined property _appState in Controller\Soap.php on line 104

• Fix method Magento\Webapi\Model\Soap\Fault::toXml() invoked with 1 parameter, 0 required in Soap\FaultTest.php on line 128

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)

[magento-engcom-team]

Hi @mhauri. Thank you for your contribution
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[magento-engcom-team]

Hi @mhauri. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.3.0 release.