Create and merge array from (multi-valued) attributes

[mrvanes]

When consuming attributes from headers instead of env, multi-valued attributes are sometimes header name postfixed (_01, _02) or merged using a separator. The former can not easily be solved without extra logic, but the latter can be exploded using the separator character. The advantage is that everything will now be cast into an array, so that values can always be array_merged. Even multi separator-merged-multi-valued attributes will result in the correct multi-valued value this way.

This will solve #293

TODO: I lack the time and Nextcloud app development experience to introduce a separator setting in UI.
Please add one before merging this PR.
Just make sure to replace an empty separator "" with "\0" so that explode still works and strings aren't unexpectedly cut into smaller pieces. \0 should not naturally occur in strings.

[tuxcrafter]

What lines can I add to the code to print some debug information.

My saml is passing an array of groups, but I only get one group in my user preferences?

[Tue Mar 12 23:45:11.871725 2019] [:error] [pid 3848] [12/Mar/2019:23:45:11]  DEBUG(ipsilon/providers/common.py:93 Redirect.debug()): saml2: jelledj's attributes: {'_groups': ['generalinformation', 'communications'], 'surname': 'de Jong', '_extras': {'sssd': {}}, 'groups': ['generalinformation', 'communications'], '_auth_type': 'password', 'givenname': 'Jelle', 'email': '[email protected]'}
[Tue Mar 12 23:45:11.873282 2019] [:error] [pid 3848] [12/Mar/2019:23:45:11]  DEBUG(ipsilon/providers/common.py:93 Redirect.debug()): saml2: value de Jong
[Tue Mar 12 23:45:11.874799 2019] [:error] [pid 3848] [12/Mar/2019:23:45:11]  DEBUG(ipsilon/providers/common.py:93 Redirect.debug()): saml2: value generalinformation
[Tue Mar 12 23:45:11.876352 2019] [:error] [pid 3848] [12/Mar/2019:23:45:11]  DEBUG(ipsilon/providers/common.py:93 Redirect.debug()): saml2: value communications
[Tue Mar 12 23:45:11.877856 2019] [:error] [pid 3848] [12/Mar/2019:23:45:11]  DEBUG(ipsilon/providers/common.py:93 Redirect.debug()): saml2: value Jelle
[Tue Mar 12 23:45:11.879394 2019] [:error] [pid 3848] [12/Mar/2019:23:45:11]  DEBUG(ipsilon/providers/common.py:93 Redirect.debug()): saml2: value [email protected]