tool_consumer_instance_guid is required when it should be optional

[xow]

In the specification, tool_consumer_instance_guid is only listed as recommended. https://www.imsglobal.org/specs/ltiv2p0/implementation-guide#toc-42 However in the provider library it is required. https://github.com/IMSGlobal/LTI-Tool-Provider-Library-PHP/blob/master/src/ToolProvider/ToolProvider.php#L812

This means (for example) SAKAI does not work with tools using this library.

[mdjnelson]

Created pull request #47 to fix this using David's commit as David's PR was done against his own fork.