couple of bugfixes

See also tarantool/cartridge@81dabfb

```
1. Improve graphql tests

2. Make "isValueOfTheType" required field for scalar types

In order to avoid possible misuse. It will throw an error anyway.
This infact moves such check from runtime[1] to declaration time.

Closes tarantool/cartridge#854

[1] https://github.com/tarantool/cartridge/blob/a9165b69af471e2c9d7ee1dce516c62c8389b9e8/cartridge/graphql/validate_variables.lua#L98

3. graphql: fix possible type mismatch between field type and variable type

Closes tarantool/cartridge#853
```

Author: olegrok

graphql/rules.lua

@@ -4,6 +4,10 @@ local util = require(path .. '.util')
 local introspection = require(path .. '.introspection')
 local query_util = require(path .. '.query_util')
 
+local function error(...)
+  return _G.error(..., 0)
+end
+
 local function getParentField(context, name, count)
   if introspection.fieldMap[name] then return introspection.fieldMap[name] end
 
@@ -517,6 +521,19 @@ local function isVariableTypesValid(argument, argumentType, context,
         'is not compatible with the argument type "%s"'):format(variableName,
         util.getTypeName(variableType), util.getTypeName(argumentType))
     end
+  elseif argument.value.kind == 'list' then
+    -- find variables deeper
+    local parentType = argumentType
+    if parentType.__type == 'NonNull' then
+      parentType = parentType.ofType
+    end
+    local childType = parentType.ofType
+
+    for _, child in ipairs(argument.value.values) do
+      local ok, err = isVariableTypesValid({value = child}, childType, context,
+              variableMap)
+      if not ok then return false, err end
+    end
   elseif argument.value.kind == 'inputObject' then
     -- find variables deeper
     for _, child in ipairs(argument.value.values) do

graphql/types.lua

@@ -77,6 +77,7 @@ end
 function types.scalar(config)
   assert(type(config.name) == 'string', 'type name must be provided as a string')
   assert(type(config.serialize) == 'function', 'serialize must be a function')
+  assert(type(config.isValueOfTheType) == 'function', 'isValueOfTheType must be a function')
   if config.parseValue or config.parseLiteral then
     assert(
       type(config.parseValue) == 'function' and type(config.parseLiteral) == 'function',
@@ -366,13 +367,22 @@ types.boolean = types.scalar({
   end,
 })
 
+--[[
+The ID scalar type represents a unique identifier,
+often used to refetch an object or as the key for a cache.
+The ID type is serialized in the same way as a String;
+however, defining it as an ID signifies that it is not intended to be human‐readable.
+--]]
 types.id = types.scalar({
   name = 'ID',
   serialize = tostring,
   parseValue = tostring,
   parseLiteral = function(node)
     return node.kind == 'string' or node.kind == 'int' and node.value or nil
-  end
+  end,
+  isValueOfTheType = function(value)
+    return type(value) == 'string'
+  end,
 })
 
 function types.directive(config)