ME: updated tests to check for 'Last-Modified' Header property
ME: added 'Last-Modified' Header property to the viewsets
ME: added functions to get formatted header date and to get date based on args
ME: fixed bug causing incorrect field_xpath and updated test
fixed 'NoneType' error causes by connect and user_profile viewsets
ME: refactored 'get_date' and added 'last_modified_header' and 'merge_dicts' functions
ME: refactored code; implemented 'last_modified_header', 'get_date' and 'merge_dicts' functions is viewsets
Merge pull request #696 from onaio/619-enable-last-modified-header-on-api-outputs

619 enable last modified header on api outputs