The new sharing code handles things differently so there is no way for
create shares to handle this all properly.

* Small changes plus check of the file using propfind
* Restaured line removed accidentally

Check fields of the share response

Added missing fields to check if their values are numbers

Added tests about getting shares of a file and a test for deleting a user from a group which doesn't exist

added logic necessary for preconditions and postconditions of sharing tests

Added sharing by link tests and sharing with group

Added test which reproduces issue 19950 (adding expiration date)

refactored given and then statements to be preconditions and not operations