Type union coercion fails when record types come before others

[dacreify]

Summary

The match-clj? implementation on RecordType doesn't check if clj-map is actually a map, which it may not be if part of a type union.

Repro

1. Go into jackdaw.serdes.avro-test/schema-type-test to the (testing "union" form
2. In the avro-schema binding move record-1-schema so that it's the first element of the vector
3. Run the test and observe that it fails