convert :null to nil args value before method executing

[0xAX]

After execution of a method we convert nil values to null to represent
null values in json in a correct way. We also need to convert null
values to nil before execution of a method to represent correctly
these values in database.

[coveralls]

Coverage increased (+0.4%) to 64.251% when pulling d013482 on 0xAX:convert-nul-to-nil into 1550dcf on xerions:master.

[coveralls]

Coverage increased (+0.4%) to 64.251% when pulling 466d83c on 0xAX:convert-nul-to-nil into 1550dcf on xerions:master.

[coveralls]

Coverage increased (+0.4%) to 64.251% when pulling 16723d3 on 0xAX:convert-nul-to-nil into 1550dcf on xerions:master.