Method `getValue` wasn’t working for textarea’s

Method `getValue` wasn't returning actual textarea value for quite a long time. Bug was never discovered, because of `testAdvancedForm` test wasn't checking textareas at all (see https://github.com/Behat/Mink/pull/403).

I've fixed `getValue` method and also replaced manual type conversion (e.g. “string:”, “boolean:”, etc) with `JSON.stringify`.

I guess original code dates back to PHP 4.4, where there was no `json_decode` function.