primaryAttributeKey check for string

[asifrahman001]

If tag[primaryAttributeKey] is not string, it should return false. Since, In many cases we have tag[primaryAttributeKey] as object.

We get this error if the attr is an object
TypeError: tag[primaryAttributeKey].toLowerCase is not a function at /node_modules/react-helmet/lib/HelmetUtils.js:124:100 at Array.filter (<anonymous>) at /node_modules/react-helmet/lib/HelmetUtils.js:100:22 at Array.reduce (<anonymous>) at getTagsFromPropsList (/node_modules/react-helmet/lib/HelmetUtils.js:97:18) at reducePropsToState (/node_modules/react-helmet/lib/HelmetUtils.js:181:19)

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