Fixes test `DeserializeScalarEdgeCases` for non english cultures.

[jhm-ciberman]

Summary

The test DeserializeScalarEdgeCases fails if the machine culture does not use the dot decimal separator. This is caused due to object.ToString() being culture sensitive for floats and doubles.

This PR addresses partially #792. For a full explanation refer to that issue.

Fix

This PR fixes the test using string.Format with CultureInfo.InvariantCulture instead of object.ToString()