setValidateDefaults=false does not seem to work

Followup to AVRO-2035.

I am testing 1.9 SNAPSHOT. I set `Schema.Parser.setValidateDefaults` to false, and still get a failure at compile-time if the default does not match the schema. For example, a field definition of:

{"name": "name", "type": "string", "default": null}

Should this definition succeed if `setValidateDefaults(false)` is called? If not, should the `setValidateDefaults` method just be removed?

The exception is:

Caused by: org.apache.avro.AvroTypeException: Invalid default for field name: null not a

{"type":"string","avro.java.string":"String"}

at org.apache.avro.Schema.validateDefault(Schema.java:1482)
at org.apache.avro.Schema.access$300(Schema.java:84)
at org.apache.avro.Schema$Field.<init>(Schema.java:493)
at org.apache.avro.Schema$Field.<init>(Schema.java:485)
at org.apache.avro.Schema$Field.<init>(Schema.java:504)
at org.apache.avro.compiler.specific.SpecificCompiler.addStringType(SpecificCompiler.java:689)
at org.apache.avro.compiler.specific.SpecificCompiler.addStringType(SpecificCompiler.java:668)
at org.apache.avro.compiler.specific.SpecificCompiler.compile(SpecificCompiler.java:598)
at org.apache.avro.compiler.specific.SpecificCompiler.compileToDestination(SpecificCompiler.java:527)