[Process] fix issue where env contains array vals

See ticket #7196
There are cases where $env or $_ENV can contain a value that is an array
This will cause Process to throw an Array to String conversion exception
Initially I submitted a patch of Process.php, however Fabien indicated
that it shouldn't be fixed there (see below pull request).

Before recently, a simple work around would be to set:

  register_argc_argv = On

in php.ini, however with recent changes, this seems to no longer work.

Original pull request: https://github.com/symfony/symfony/pull/7354
Merge branch 'ticket_7196' of github.com:mmucklo/symfony into ticket_7196