VizierClass no longer respects the order of columns requested

VizierClass used to return the columns in the order that the caller requested them, but does not any more. In the example below, the columns requested are `['GWGC', '_RAJ2000', '_DEJ2000', 'Dist']`. In astroquery 0.4.1, they are returned in that order. However, in astroquery 0.4.2, they are returned in a different order.

## Expected (old) behavior, astroquery 0.4.1

```pycon
>>> from astroquery.vizier import VizierClass
>>> vizier = VizierClass(columns=['GWGC', '_RAJ2000', '_DEJ2000', 'Dist'])
>>> cat, = vizier.get_catalogs('VII/281/glade2')
>>> cat

GWGC _RAJ2000 _DEJ2000 Dist
deg deg Mpc
str28 float64 float64 float64
--------- -------------------- -------------------- --------------------
NGC0253 11.8880600000000012 -25.2887990000000045 3.92595099046000
NGC5128 201.3656459999999981 -43.0187110000000104 3.76743399832000
--- 56.7021410000000046 68.0961070000000177 8.35574783250000
NGC5236 204.2529999999999859 -29.8654999999999973 4.46732076201000
NGC4736 192.7214509999999734 41.1201519999999974 4.24571533533000
NGC0055 3.7233399999999999 -39.1966290000000015 2.18817755456000
... ... ... ...
--- 186.0309599999999932 6.6074700000000002 33.59537004350000
IC3652 190.2441099999999778 11.1844080000000012 22.44885791180000
--- 188.2000119999999868 37.6218029999999999 51.65597412030000
NGC4328 185.8335109999999872 15.8205450000000045 8.08313872063000
PGC042497 190.2097319999999741 4.5256780000000010 11.11130436180000
PGC045084 195.8197329999999852 -17.4230309999999982 6.91790412822000
```

## Actual (new) behavior, astroquery 0.4.2

```pycon
>>> from astroquery.vizier import VizierClass
>>> vizier = VizierClass(columns=['GWGC', '_RAJ2000', '_DEJ2000', 'Dist'])
>>> cat, = vizier.get_catalogs('VII/281/glade2')
>>> cat

_RAJ2000 _DEJ2000 GWGC Dist
deg deg Mpc
float64 float64 str28 float64
-------------------- -------------------- --------- --------------------
11.8880600000000012 -25.2887990000000045 NGC0253 3.92595099046000
201.3656459999999981 -43.0187110000000104 NGC5128 3.76743399832000
56.7021410000000046 68.0961070000000177 --- 8.35574783250000
204.2529999999999859 -29.8654999999999973 NGC5236 4.46732076201000
192.7214509999999734 41.1201519999999974 NGC4736 4.24571533533000
3.7233399999999999 -39.1966290000000015 NGC0055 2.18817755456000
... ... ... ...
186.0309599999999932 6.6074700000000002 --- 33.59537004350000
190.2441099999999778 11.1844080000000012 IC3652 22.44885791180000
188.2000119999999868 37.6218029999999999 --- 51.65597412030000
185.8335109999999872 15.8205450000000045 NGC4328 8.08313872063000
190.2097319999999741 4.5256780000000010 PGC042497 11.11130436180000
195.8197329999999852 -17.4230309999999982 PGC045084 6.91790412822000
```

Reproduce the ordering difference with VizierClass(columns=['GWGC', '_RAJ2000', '_DEJ2000', 'Dist']) and get_catalogs('VII/281/glade2'). Start by tracing how VizierClass handles requested columns and compare the 0.4.1 and 0.4.2 behavior. Done means returned tables preserve the caller's column order and a regression test covers this example.

Written by the indexing model from the issue text.