Micro optimize setCustomAttribute call.

[tr3mulant]

Micro optimization for CustomAttributeTrait.

The setCustomAttribute method had two fetches for the database for getting the custom attribute when the $attr wasn't a model and the attribute existed.

Now, the method just tries to resolve the attribute object by called resolveCustomAttributeObject in 3 ways.

1. Early return if we already have a CustomAttribute
2. Attempt to find it via the relationship in the method getCustomAttribute (which just actually checks if there is a CustomAttributeValue for the item tied to the CustomAttribute in question)
3. Last resort will just fetch the CustomAttribute from the actual table

[dvicklund]

Seems like a reasonable change - the hasCustomAttribute function is nonsense anyway - no need to get a model to see if it exists, then subsequently getCustomAttribute to return the actual model to the caller. Much better to minimize redundant calls. Merging