`parseValue` expects get an instance of `Upload` value
Hi there.
I cannot upload a file as I get "Upload value invalid" error, unless I override `parseValue` function. As I see, there is a [condition](https://github.com/jaydenseric/graphql-upload/blob/73ef939ef16a40358eb80e0a54c5c3ead725b2fe/lib/GraphQLUpload.js#L65) in `parseValue` function (in the `GraphQLUpload.js` file) that expects to get an instance of [`Upload`](https://github.com/jaydenseric/graphql-upload/blob/73ef939ef16a40358eb80e0a54c5c3ead725b2fe/lib/Upload.js#L14) `value`.
I think this condition should be changed to something like this `value instanceof Blob` to resolve the error (Check the `value` to be an instance of Blob instead of `Upload`).

p.s.: It seems this condition added to resolve [this issue](https://github.com/jaydenseric/graphql-upload/issues/175).