add check for in_signal_handler condition

[vadim-ex]

Some design considerations:

• there is no clean way to check whether we are in signal handling context, so it needs to be passed as parameter.
• the right way (and, hopefully, one day available) is to pass it as a keyword parameter. Except that we are still supporting 1.9.x Ruby
• when switching to keyword would occur, the defaults would disappear from method definitions (and ugly in_signal_handler=in_signal_handler as well. It is also a bit safer since some existing methods use parameters with default.

[myronmarston]

LGTM

[vadim-ex]

Thank you!