Call getHeaders() to load all groups, rather than expecting a string of groups with a delimiter

[mamccorm]

I'm not sure if this is something that the maintainers will want to merge, however we faced an issue when using this plugin with another third party application.

It seems that the plugin expects to receive a single string of groups, with a pipe '|' as a delimiter. We're not quite sure why it was implemented this way, but it doesn't seem to be the standard way that groups would be passed in via the headers.

We've forked the plugin and made this change. Thought it worth highlighting via PR in case it was something you wanted to consider re-factoring / merging in.

The changes in this PR replace getHeader() with a call to getHeaders().

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