[#504] Fix missing question settings

Correcting the copy of survey questions from one instance to another
where some settings were missing
[#504] Take into account "null" string for Boolean fields

In cases where a boolean value is not set in json response, i.e. "null" string is
returned as response, ensure that corresponding field is also set to null
Merge pull request #532 from akvo/feature/504

[#504] Fix for missing data when transferring surveys
Merge branch 'develop' of github.com:akvo/akvo-flow into tests/524_clustering