Issue #1213 Fixing issue with Sharing plugin

Json_Controller::calculate_center() expencting marker to be an object but
array being passed int by Share_Controller::index(). Not sure if an object
is ever passed to calculate_center(), but this update shouldn't cause a
problem as object properties are acessible using array syntax anyway.
Merge pull request #1218 from ditorelo/1213b

Issue #1213 Fixing issue with Sharing plugin