Fix incorrect number of columns about memory when outputting to a csv

With '-m' option, the number of columns seems wrong when outputting to a csv file.
'self.nick' and 'self.vars' are not symmetrical. 

```bash
$ dstat -m --output test.csv
------memory-usage-----
 used  free  buff  cach
3578M  243M  551M  166M
3578M  243M  551M  166M^C
$ cat test.csv
"Dstat 0.7.2svn CSV output"
"Author:","Dag Wieers <dag@wieers.com>",,,,"URL:","http://dag.wieers.com/home-made/dstat/"
"Host:","labkfub",,,,"User:","root"
"Cmdline:","dstat -m --output test.csv",,,,"Date:","11 Jun 2014 22:45:31 JST"
"memory usage",,,
"used","free","buff","cach"
3751710720,255250432,577626112,173756416,2630819840,2572288,116830208 #<- It seems wrong
3751710720,255221760,577650688,173756416,2630823936,2572288,116830208 #<- It seems wrong

$ dstat -mc --output test2.csv
------memory-usage----- --total-cpu-usage--
 used  free  buff  cach|usr sys idl wai stl
3578M  238M  556M  166M|  0   0 100   0   0
3578M  238M  556M  166M|  0   0 100   0   0^C
$ cat test2.csv
"Dstat 0.7.2svn CSV output"
"Author:","Dag Wieers <dag@wieers.com>",,,,"URL:","http://dag.wieers.com/home-made/dstat/"
"Host:","labkfub",,,,"User:","root"
"Cmdline:","dstat -mc --output test2.csv",,,,"Date:","11 Jun 2014 22:53:12 JST"
"memory usage",,,,"total cpu usage",,,,
"used","free","buff","cach","usr","sys","idl","wai","stl"
3751710720,249569280,583233536,173776896,2630905856,2592768,116817920,0.013,0.012,99.949,0.024,0.002 #<- It seems wrong seriously
3751710720,249532416,583266304,173776896,2630909952,2592768,116817920,0,0,100,0,0 #<- It seems wrong seriously
```