Pass NOP generator to exploit_simple

This also makes exploit_simple consistent with its documentation.

lib/msf/base/simple/exploit.rb

@@ -98,6 +98,11 @@ module Exploit
       # best encoder.
       exploit.datastore['ENCODER'] = opts['Encoder'] if opts['Encoder']
 
+      # Use the supplied NOP generator, if any.  If one was not specified, then
+      # nil will be assigned causing the exploit to default to picking a
+      # compatible NOP generator.
+      exploit.datastore['NOP'] = opts['Nop'] if opts['Nop']
+
       # Force the payload to share the exploit's datastore
       driver.payload.share_datastore(driver.exploit.datastore)