Fix #140

[anaxamaxan]

Use getMorphClass() instead of get_class to resolve the taggable_type. Add same to the anonymous class in getQualifiedPivotTableName().

Thank you for helping to make this package better!

Please make sure you've read CONTRIBUTING.md before submitting your pull request, and that you have:

• [x] provided a rationale for your change (I try not to add features that are going to have a limited user-base)

See issue #140 for info.

• [x] used the PSR-2 Coding Standard

• [ ] added tests No change to tests. I don't know how to change Relation::enforceMorphMap() dynamically.

• [ ] documented any change in behaviour (e.g. updated the README.md, etc.) No change to behavior.

• [x] only submitted one pull request per feature It's a small change.