[5.x] Fix error when serializing eloquent query builders

[duncanmcclean]

This pull request fixes an issue with the {{ nocache }} tag and Statamic's EloquentQueryBuilder, where passing eloquent query builder instances to partials would cause an error to be thrown:

Serialization of 'PDO' is not allowed

This error was happening due to the PDO object, which is nested in Eloquent's query builder not being serializable. This PR fixes that by removing the connection and grammar properties from the query before serializing, then sets them again when unserializing.

Fixes statamic/eloquent-driver#291.

[jasonvarga]

This removes the builder when serializing, but does the query still work once its unserialized?

[duncanmcclean]

This removes the builder when serializing, but does the query still work once its unserialized?

Ah, you're right. It didn't.

I've refactored the fix so it removes the connection & grammar properties on the query before serialising, then sets them again when unserializing.

[jasonvarga]

This is kinda just masking the problem. If you had a regular Laravel query builder instance you'd get the same error. But I'm okay with merging anyway.