Current logic present in DefinitionGenerator#processUnionType has no clue about the type of default value anyway.

What is done there is:

• check for string, boolean or number type in a cycle
• as a consequence we can have as a result string, boolean and number at the same time
• what is correct because that the nature of union
• then we are trying to cast default value to some type preferring string - what is not correct

I propose to delete DefinitionGenerator#processUnionType, use first member type for generating example and omitting default value at all.