AvroCoder should be able to handle anonymous classes as schemas

Currently, the determinism checker NPEs with:
java.lang.IllegalArgumentException: Unable to get field id from class null
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.getField(AvroCoder.java:710)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.checkRecord(AvroCoder.java:548)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.doCheck(AvroCoder.java:477)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.recurse(AvroCoder.java:453)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.checkRecord(AvroCoder.java:567)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.doCheck(AvroCoder.java:477)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.recurse(AvroCoder.java:453)
at com.google.cloud.dataflow.sdk.coders.AvroCoder$AvroDeterminismChecker.check(AvroCoder.java:430)
at com.google.cloud.dataflow.sdk.coders.AvroCoder.<init>(AvroCoder.java:189)
at com.google.cloud.dataflow.sdk.coders.AvroCoder.of(AvroCoder.java:144)
at mypackage.GenericsTest$1.create(GenericsTest.java:102)
at com.google.cloud.dataflow.sdk.coders.CoderRegistry.getDefaultCoderFromFactory(CoderRegistry.java:797)
at com.google.cloud.dataflow.sdk.coders.CoderRegistry.getDefaultCoder(CoderRegistry.java:748)
at com.google.cloud.dataflow.sdk.coders.CoderRegistry.getDefaultCoder(CoderRegistry.java:719)
at com.google.cloud.dataflow.sdk.coders.CoderRegistry.getDefaultCoder(CoderRegistry.java:696)
at com.google.cloud.dataflow.sdk.coders.CoderRegistry.getDefaultCoder(CoderRegistry.java:178)
at com.google.cloud.dataflow.sdk.values.TypedPValue.inferCoderOrFail(TypedPValue.java:147)
at com.google.cloud.dataflow.sdk.values.TypedPValue.getCoder(TypedPValue.java:48)