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* customOrDefaultKey is a re-useable method to determing if the developer
* using the plugin has defined a custom key for extractin the JWT
* @param {Object} options - the options passed in when registering the plugin
* @param {String} key - name of the key e.g `urlKey` see:
* @param {String} _default - the default key used if no custom is defined.
* @returns {String} key - the custom key or default key.
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