Fix "default": undefined

[mkwtys]

{"default": undefined} in this 3 file.

test/syntax/idl/dictionary-inherits.widl
test/syntax/idl/dictionary.widl
test/syntax/idl/overloading.widl

tried this assertion in the test/syntax.js.

assert.deepEqual(wp.parse(fs.readFileSync(idl, "utf8"), opt), JSON.parse(fs.readFileSync(json, "utf8")));