Fix: properties of data source were not reset when changing type.
Fix: properties of data source were not reset when changing type.

Fixes #1748.

denisov-vlad/redash,useabode/redash,moritz9/redash,useabode/redash,hudl/redash,getredash/redash,crowdworks/redash,chriszs/redash,chriszs/redash,chriszs/redash,hudl/redash,alexanderlz/redash,44px/redash,rockwotj/redash,EverlyWell/redash,moritz9/redash,rockwotj/redash,crowdworks/redash,rockwotj/redash,hudl/redash,hudl/redash,alexanderlz/redash,EverlyWell/redash,44px/redash,vishesh92/redash,EverlyWell/redash,useabode/redash,crowdworks/redash,rockwotj/redash,vishesh92/redash,denisov-vlad/redash,moritz9/redash,EverlyWell/redash,denisov-vlad/redash,44px/redash,getredash/redash,useabode/redash,crowdworks/redash,alexanderlz/redash,vishesh92/redash,getredash/redash,vishesh92/redash,chriszs/redash,moritz9/redash,getredash/redash,44px/redash,denisov-vlad/redash,getredash/redash,alexanderlz/redash,denisov-vlad/redash