Fix required: Fix plugin Dynamic type with sub field of type "input_group" to display default values and save in DB

Same issue as before, except this bug is for when field of type input_group is used within field of type array:array:string
(See bug report: http://forums.b2evolution.net/6-9-2-bug-adding-form-color-field with fix: https://github.com/b2evolution/b2evolution/commit/29144d873f33a85b26f9153d260b8d029510dc6b and https://github.com/b2evolution/b2evolution/commit/4fc3241ebea1ff3b1d6bab50865370243cea8ba2)

This report is with following fix implemented: https://github.com/b2evolution/b2evolution/commit/b1520f5153de9f82996cc001bb6e4d496ba78b2b