PAssert doesn't use the correct Coder

Run into issue with below test, that it can pass although I'm sure it should fail. It seems `PAssert` doesn't use the right Coder to compare expected/actual values.

See test case here [https://github.com/XuMingmin/beam/blob/5d2bc2aa7c344b8b9adf4ceeac2d7450d6ddec1e/sdks/java/extensions/sql/src/test/java/org/apache/beam/sdk/extensions/sql/BeamSqlDslUdfUdafTest.java#L93-L104](https://github.com/XuMingmin/beam/blob/5d2bc2aa7c344b8b9adf4ceeac2d7450d6ddec1e/sdks/java/extensions/sql/src/test/java/org/apache/beam/sdk/extensions/sql/BeamSqlDslUdfUdafTest.java#L93-L104)

Imported from Jira [BEAM-3752](https://issues.apache.org/jira/browse/BEAM-3752). Original Jira may contain additional context.
Reported by: mingmxu.

Start with the linked BeamSqlDslUdfUdafTest.java test at lines 93-104 and run it to reproduce the case where the assertion passes unexpectedly. Trace how PAssert compares the expected and actual values and how their Coders are selected; done means the test fails when those values differ according to the correct Coder.

Written by the indexing model from the issue text.