Merge pull request #17 from layerhq/develop

Fix nulls for non-collection types
Merge pull request #18 from layerhq/develop

Use the types from prepare for execute
Fix few things after merging #18
Don't (re)define the set() type just in case
Add property tests
Merge pull request #20 from puzza007/po-prop-tests

Add property tests for type conversions
Add a test for executing prepared statement