LOG_FIRST_N and similar macros may fail to compile after case label

[eelstork]

Since a case label does not imply a new scope, compilers may emit an error (example) when LOG_FIRST_N and similar macros are used after case inside a switch statement.
I am suggesting a fix, see #71.

[ukai]

write LOG_FIRST_N in block like

  case:
    {
       LOG_FIRST_N(INFO, 20) << "...";
    }