Fix test related to default negative values for 4.2

- Rails no longer type casts default value at the time of column
  initialization
  https://github.com/rails/rails/commit/4d3e88fc757a1b6f6c468418af01dca677e41edf.
- Also the default attribute of Column instance is not public. The
  public API is column_defaults method.
- Updated test to use this method for AR > 4 and fallback to default for
  older versions.