[5.x] Prepare value & operator before passing to Eloquent Query Builder

[duncanmcclean]

This pull request partially fixes statamic/eloquent-driver#424, by ensuring that the value and operator are handled properly before being passed to the Eloquent Query Builder.

The Problem

Currently, when you're storing entries in the database, attempting to query a column with a value matching an operator will result in an error:

Entry::query()->where('site', 'is')->get();

InvalidArgumentException 
Illegal operator and value combination. 
at vendor/laravel/framework/src/Illuminate/Database/Query/Builder.php:945

The query works as expected when querying the model directly, or when using the Stache query builder. is simply becomes the value when a third argument hasn't been provided.

This fix goes alongside statamic/eloquent-driver#425 which adjusts how the EloquentQueryBuilder::where() method is called.