throw if it's impossible to coerce boolean value

Graphql spec states:
> Input Coercion
When expected as an input type, only boolean input values are accepted.
All other input values must raise a request error indicating an incorrect type.

(https://spec.graphql.org/draft/#sec-Boolean)

So we can't coerce string, numeric and enum values. And here we
faced main issue - because graphql parser considered "False" and
"True" as enum values (correct boolean values is "true" and
"false"). And if user passed "False" value to boolean argument it
silently converted to true. This patch check that passed node type
is boolean and raises if it's not so.

Closes #14

Author: olegrok

graphql/types.lua

@@ -382,6 +382,9 @@ types.boolean = types.scalar({
   description = "The `Boolean` scalar type represents `true` or `false`.",
   serialize = coerceBoolean,
   parseLiteral = function(node)
+    if node.kind ~= 'boolean' then
+      error(('Could not coerce value "%s" with type "%s" to type boolean'):format(node.value, node.kind))
+    end
     return coerceBoolean(node.value)
   end,
   isValueOfTheType = isBoolean,

test/unit/graphql_test.lua

@@ -1028,3 +1028,32 @@ function g.test_types_for_different_schemas()
     t.assert_error_msg_contains('Field "string_2" is not defined on type "Object"',
             validate, schema_1, parse([[query { object_list { long_1 string_2 } }]]))
 end
+
+function g.test_boolean_coerce()
+    local query = types.object({
+        name = 'Query',
+        fields = {
+            test_boolean = {
+                kind = types.boolean.nonNull,
+                arguments = {
+                    value = types.boolean,
+                    non_null_value = types.boolean.nonNull,
+                }
+            },
+        }
+    })
+
+    local test_schema = schema.create({query = query})
+
+    validate(test_schema, parse([[ { test_boolean(value: true, non_null_value: true) } ]]))
+    validate(test_schema, parse([[ { test_boolean(value: false, non_null_value: false) } ]]))
+    validate(test_schema, parse([[ { test_boolean(value: null, non_null_value: true) } ]]))
+
+    -- Errors
+    t.assert_error_msg_contains('Could not coerce value "True" with type "enum" to type boolean',
+            validate, test_schema, parse([[ { test_boolean(value: True) } ]]))
+    t.assert_error_msg_contains('Could not coerce value "123" with type "int" to type boolean',
+            validate, test_schema, parse([[ { test_boolean(value: 123) } ]]))
+    t.assert_error_msg_contains('Could not coerce value "value" with type "string" to type boolean',
+            validate, test_schema, parse([[ { test_boolean(value: "value") } ]]))
+end