fix(gatsby-source-mongodb): work with _id types that are not ObjectIds

[senorpedro]

Description

Using mongodb, it is possible to use arbritary values (e.g. strings) as _id.
In that case the current implementation will throw
"TypeError: _id.toHexString is not a function"
This fix will prevent this, at least for string _ids.

[sidharthachatterjee]

I hate that Mongo supports arbitrary data types for _id but this seems like a fair change to handle edge cases like those.

Thank you for patching this @senorpedro 👍

[sidharthachatterjee]

👍

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Thanks again!

[senorpedro]

I hate that Mongo supports arbitrary data types for _id but this seems like a fair change to handle edge cases like those.

Thank you for patching this @senorpedro 👍

You're welcome! Thanks for creating such awesome software! :-)