Fix nil-union validate

Closes #113

Author: Khatskevich

avro_schema/frontend.lua

@@ -743,7 +743,7 @@ copy_data = function(stack, schema, data, visited)
     -- Due to this technique, a error message is often misleading,
     -- e.x. "attempt to perform arithmetic on a string value". Unless
     -- a message starts with '@', we replace it (see copy_data_eh).
-    if schema.nullable and (data == null or data == nil) then
+    if schema.nullable and (data == nil) then
         return null
     end
     if     schematype == 'null' then
@@ -832,6 +832,8 @@ copy_data = function(stack, schema, data, visited)
     else
         stack.push(schema, data)
         local frame_no = stack.len
+        -- Replace nil -> NULL to allow it to be a key in a table.
+        data = data ~= nil and data or null
         if visited[data] then
             error('@Infinite loop detected in the data', 0)
         end

test/api_tests/var.lua

@@ -5,7 +5,7 @@ local msgpack = require('msgpack')
 
 local test = tap.test('api-tests')
 
-test:plan(53)
+test:plan(55)
 
 test:is_deeply({schema.create()}, {false, 'Unknown Avro type: nil'},
                'error unknown type')
@@ -387,5 +387,13 @@ test:is_deeply(compiled.get_types(),
         "union_type","union_value", "array*","map","fixed*"},
     "compiled.get_types")
 
+-- gh-113: nil and box.NULL are treated differently in validate of the union
+local ok, handle = schema.create({'int', 'null'})
+local ok, data = schema.validate(handle, nil)
+assert(ok, data)
+test:is(data == nil and type(data) == 'cdata', true, 'null returned')
+local ok, data = schema.validate(handle, msgpack.NULL)
+test:is(data == nil and type(data) == 'cdata', true, 'null returned')
+
 test:check()
 os.exit(test.planned == test.total and test.failed == 0 and 0 or -1)