CRM-14986 fix DB error on date find + save

this is manefesting in the failure to save contributions, however, it has been a recurrent problem
that the format used for date fields on ->find does not match the requirement of ->save
hence we see in the ContributionRecur class 3 lines like
->modified_date = CRM_Utils_Date::isoToMysql(->modified_date);
which exist simply to format the date fields for re-save.
The end_date & next_scheduled_date were missed - resulting in fatals when this function
is used
It seems kind of crazy to keep solving this problem in the form & BAO layer
so this commit is tests only & there is a commit on the packages dir
that alters the DAO->update fn to check for the presence of a '-' & do the isoToMysql there
instead.