Optional space after comma

resolves #513
diff --git a/src/lib_json/json_writer.cpp b/src/lib_json/json_writer.cpp
index a0edff4..1bb3b21 100644
--- a/src/lib_json/json_writer.cpp
+++ b/src/lib_json/json_writer.cpp
@@ -1002,7 +1002,7 @@
       if (!indentation_.empty()) *sout_ << " ";
       for (unsigned index = 0; index < size; ++index) {
         if (index > 0)
-          *sout_ << ", ";
+          *sout_ << ((!indentation_.empty()) ? ", " : ",");
         *sout_ << childValues_[index];
       }
       if (!indentation_.empty()) *sout_ << " ";