fix set_rhosts behavior/bugs.

msf  exploit(rails_xml_yaml_code_exec) > hosts

Hosts
=====

address     mac                name          os_name            os_flavor  os_sp  purpose  info  comments
-------     ---                ----          -------            ---------  -----  -------  ----  --------
10.0.0.105  00:0C:29:59:65:08  VMWIN2000SP4  Microsoft Windows                    client

msf  exploit(rails_xml_yaml_code_exec) > hosts -R

Hosts
=====

address     mac                name          os_name            os_flavor  os_sp  purpose  info  comments
-------     ---                ----          -------            ---------  -----  -------  ----  --------
10.0.0.105  00:0C:29:59:65:08  VMWIN2000SP4  Microsoft Windows                    client

RHOSTS => 10.0.0.105

msf  exploit(rails_xml_yaml_code_exec) > exit