Restore default value memcached.sess_prefix = "memc.sess.key."

Resolves #340

Author: sodabrew

Diff for: php_memcached.c

@@ -352,7 +352,7 @@ PHP_INI_BEGIN()
 	MEMC_SESSION_INI_ENTRY("connect_timeout",        "0",          OnUpdateLongGEZero,     connect_timeout)
 	MEMC_SESSION_INI_ENTRY("sasl_username",          "",           OnUpdateString,         sasl_username)
 	MEMC_SESSION_INI_ENTRY("sasl_password",          "",           OnUpdateString,         sasl_password)
-	MEMC_SESSION_INI_ENTRY("prefix",                 "memc.sess.", OnUpdateSessionPrefixString,         prefix)
+	MEMC_SESSION_INI_ENTRY("prefix",                 "memc.sess.key.", OnUpdateSessionPrefixString,         prefix)
 	MEMC_SESSION_INI_ENTRY("persistent",             "0",          OnUpdateBool,           persistent_enabled)
 
 	/* Deprecated */