Fixed issue with authentication using API token and Powershell version 6+

[sebmaurer]

Description

Just a quickfix for authentication with Confluence via API token when using Powershell version 6+.
It is required by Microsoft Web-Request to provide the header "Token" with a secure string of the corresponding token.
Microsoft Web-Request does not expect a header named "PersonalAccessToken" therefore removed this before calling it.

Motivation and Context

closes #208

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• I have added Pester Tests that describe what my changes should do.
• I have updated the documentation accordingly.

[sebmaurer]

Personally I would also recommend using SecureString throughout the whole module for the API token because in my workaround this transformation only happens in the middle of execution.