[FIX] fix converter forms to json

[tinkoff-dwh]

What is this PR for?

Fix converter for forms

What type of PR is it?

[Bug Fix]

How should this be tested?

jdbc

1. select '${checkbox:name=1|2,first|second}'

Screenshots (if appropriate)

before

after

Questions:

• Does the licenses files need update? no
• Is there breaking changes for older versions? no
• Does this needs documentation? no

[zjffdu]

Could you just use gui.toJson() in L394 ?

[felixcheung]

is there any test for this, if not, can we add one?

[tinkoff-dwh]

@zjffdu @felixcheung
done

[felixcheung]

is test failure related?

15:11:43,740  INFO org.apache.zeppelin.spark.SparkInterpreter:979 - Sending metadata to Zeppelin server: {message=Spark UI enabled, url=http://10.20.0.79:4040}
<console>:24: error: not found: value value
       def category(min: Int) = {    if (0 <= value) "error"}
                                              ^
15:11:43,772 DEBUG org.apache.zeppelin.interpreter.Interpreter:197 - key: spark.ui.enabled, value: true
15:11:43,773  INFO org.apache.zeppelin.spark.SparkInterpreter:979 - Sending metadata to Zeppelin server: {message=Spark UI enabled, url=http://10.20.0.79:4040}
defined class Person
15:11:44,112 ERROR org.apache.zeppelin.spark.SparkInterpreter:1256 - 
java.lang.NullPointerException
	at scala.tools.nsc.interpreter.IMain$ReadEvalPrint.call(IMain.scala:784)

[tinkoff-dwh]

nope

[felixcheung]

ok looks good, any more comment?

[tinkoff-dwh]

merge?