Fix: download_exec appends an extra / to request

The download_exec module parses the provided URL and appends an
unnecessary, nay--damaging I say!!!! '/' to the parsed URI. This
renders the module unusable for those who want a payload to
download and execute a file.

Before and after access.log snippets are in the redmine ticket

http://dev.metasploit.com/redmine/issues/7592

Author: rsmudge

modules/payloads/singles/windows/download_exec.rb

@@ -312,7 +312,7 @@ def generate
 	call httpopenrequest
 
 server_uri:
-	db "/#{server_uri}", 0x00
+	db "#{server_uri}", 0x00
 
 create_file:
 	jmp.i8 get_filename