Properly support slashes in Category names

[amenk]

See #194 #198

We have slashes in category names (something like Valves 1/2")

Describe the solution you'd like

In https://github.com/techdivision/import/blame/master/src/Assembler/CategoryAssembler.php#L152 we should use a CSV implode instead of a simple implode.

Describe alternatives you've considered

Removing slashes? but that is not an option :)

Additional context

If we have such a category tree

This would lead twice to the key Foo/Bar, for ID = 2 as well as ID = 3, so the array indexes collide and the ID = 2 will be overwritten.

[amenk]

@DanieliMi will try to use the csvSerializier here to add escaping