[FixRM 8482] Fix uninit constant Rex::Exploitation::JavascriptOSDetect

This fixes an uninit constant Rex::Exploitation::JavascriptOSDetect
while using a module with js_os_detect. It was originally reported
by Metasploit user @viniciuskmax

[FixRM 8482]
Land #2518, uninitialized JavascriptOSDetect fix