PHPORM-184 Use fixed key for temporary setting nested field

[GromNaN]

Fix PHPORM-184
Fix #2959

The memory leak reported in #2959 is caused by the random temporary key used before setting a nested value. There is a cache in Illuminate\Suport\Str::studly that inflate for each random key.

Also, uniqid is slow and not necessary here. The temporary key can be static as there is not risk of conflict/collision.

• The last operation of parent::setAttribute() is $this->attributes[$key] = $value;
• Then $this->attributes[$key] is read and wrote in the nested array;
• Then $this->attributes[$key] is unset.

There is no possibility of a code acting on this key in the meantime.

Checklist

• Add tests and ensure they pass
• Add an entry to the CHANGELOG.md file
• Update documentation for new features