Converting default value string of null to nil breaks legitimate schema.

[mokobrosli]

goavro/record.go

Line 112 in 8eb9f0e

if defaultValue == "null" {

This line treats "null" and null as equivalent for default values, however that is not the case. Having a default value that is actually the string "null" will fail when creating a new Codec.

To reproduce simply try goavro.NewCodec(sample) where sample is :

{ "name": "test_str", "type": [ "string", "null" ], "default": "null" }

The above would pass if the union was defined with null type as its first element but that's a different case. This would fail as well if it was a simple string type.