Lookup polymorphic type from the column before fetching the record

[st0012]

Currently, Relationship#type_for_source looks up associated record's type by:

1. Fetching its associated record.
2. Calling .class._type on that record.

This approach is inefficient because usually, a polymorphic record (the source) has _type and _id columns. And the _type column's value will be the polymorphic relationship record's type.

So this PR makes type_for_source check the type value from column before fetching the associated record.

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[lgebhardt]

@st0012 Thanks!