Fix undefined property when adding a property with a different default value.

[tbolon]

short story: a property accessor is missing an _ raising another exception and making debugging difficult.

When developing a template, I found myself stuck with a strange error about an unidentified property.

After some investigation, I found that I was adding a property with a different defaultValue than a previous one declared, but the exception was not helpful.

This fix simply add the missing character.

[bago]

Thank you!

[tbolon]

Thank you for this wonderful project :)