Values of type "double" should not use locale-specific encoding

[Grundik]

Q	A
Bug report?	yes
Feature request?	no
BC Break report?	no
RFC?	no

Steps required to reproduce the problem

1. Use locale with decimal separator other than dot, e.g. setlocale(LC_ALL, 'ru_RU.UTF-8')
2. Try to serialize value of type double with fractional part, e.g. 1.234

Expected Result

• Value should be encoded in using fixed format: no thousands separator, dot as fractional separator, e.g. "1.234"

Actual Result

• Value encoded in locale-aware way, with locale-specific settings, e.g. "1,234", which will lead to parsing errors on produced serialized file.

This behaviour caused by converting double values to string type by using (string) operator:

    public function visitDouble($data, array $type, Context $context)
    {
        ...
            return $this->document->createTextNode((string) $data);
        ...

Easiest way to convert double to machine-readable string is using json_encode: json_encode((double)$data)

[Grundik]

Fix: #1042