json: add optional member name in ToJSON() template method
json: add optional member name in ToJSON() template method

Allows to use TBufferJSON::ToJSON() also for selected data members
conversion
root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,olifre/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,root-mirror/root,olifre/root,olifre/root,olifre/root,root-mirror/root,olifre/root,olifre/root