3.1.0

• PSR-15 server request handler & middleware support
• split the non PSR methods of Psr7\Uri into a separate class Psr7\UriExtended
• Psr7\get_xml() now has a $assoc parameter to return an associative array, like its counterpart Psr7\get_json()
• additional PSR-17 integration tests
• PHPUnit 8, PHP 7.4 tests
• added backslashes in front of PHP built-in functions to improve performance
• several doc fixes