Release v1.6.0

Updates to header rules along with bug fixes and general code and test improvements

• The default value for X-XSS-Protection has been changed to '0' as per the current OWASP standard: https://github.com/OWASP/CheatSheetSeries/blob/master/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.md#x-xss-protection-header
• The default value for Cache-Control has been changed to 'no-store, max-age=0' as per the current Mozilla standard on preventing caching
• All header keys are now case-insensitive
• New functionality has been added to enable rule specification at a directive level
• Improvements to exception handling
• Improvements in tests
• Improvement in documentation