Add a validation code to check if json_column_name column is string type

[muga]

This fix is to add a validation code for checking if the type of a column specified as json_column_name option is string or not. setExpandedJsonColumns method requires that the json column is string type. It's nice to have such type validation.

To add an unit test for this validation, I removed final modifier from schema field in TestExpandJsonFilterPlugin class. But this change might not be bad for us.

[civitaspo]

ship it!

[civitaspo]

this change might not be bad for us

@muga hmm, could you comment on the code the reason the change is needed?
(excuse, I merged already this p-r.)

[muga]

@civitaspo Ok, I will do that. To be clarified, the reason why I removed final is because my test method requires different schema object from [_c0:string, _c1:string], you know. In the future, I think that new additional test methods also will require different schemas. So, this change is reasonable.

[muga]

Added comment on the test class. #10

Thank you for merging.