Fix variable validation for type as table

Before this patch, describing argument type in schema with a table
resulted in "unsupported Lua type 'function'" error on variable
type validation.

Based on PR #22 by @no1seman

Author: DifferentialOrange

graphql/rules.lua

@@ -516,6 +516,7 @@ local function isVariableTypesValid(argument, argumentType, context,
     if hasDefault and variableType.__type ~= 'NonNull' then
       variableType = types.nonNull(variableType)
     end
+    if argumentType.kind ~= nil then argumentType = argumentType.kind end
 
     if not isTypeSubTypeOf(variableType, argumentType, context) then
       return false, ('Variable "%s" type mismatch: the variable type "%s" ' ..

test/integration/graphql_test.lua

@@ -1535,3 +1535,28 @@ function g.test_schema_input_arg_described_with_kind()
     t.assert_equals(errors, nil)
 end
 
+function g.test_schema_input_arg_described_with_kind_variable_pass()
+    local function callback(_, args)
+        return args[1].value
+    end
+
+    local query_schema = {
+        ['test'] = {
+            kind = types.string.nonNull,
+            arguments = {
+                arg = {
+                    kind = types.string.nonNull,
+                },
+            },
+            resolve = callback,
+        }
+    }
+
+    local query = [[
+        query ($arg: String!) { test(arg: $arg) }
+    ]]
+    local variables = { arg = 'B' }
+
+    local _, errors = check_request(query, query_schema, nil, nil, { variables = variables })
+    t.assert_equals(errors, nil)
+end