vtgate: Fix that the tablet type is shown in the "Partitions:" list on the status page.
vtgate: Fix that the tablet type is shown in the "Partitions:" list on the status page.

Before, string(i) was shown where i is the index of the array element (e.g. 0, 1 or 2). string(i) for these low i values always returned an empty string because it would be interpreted as an ASCII code and these low values do not map to printable characters.

Other changes:
- use upper case tablet type string in the ServedFrom output. This is consistent with the "Gateway Status" and "Health Check Cache" output.
- Fixed output of the ShardingColumnType enum.
HubSpot/vitess,sougou/vitess,mapbased/vitess,rnavarro/vitess,rnavarro/vitess,mapbased/vitess,sougou/vitess,alainjobart/vitess,tinyspeck/vitess,vitessio/vitess,mahak/vitess,applift/vitess,davygeek/vitess,alainjobart/vitess,tinyspeck/vitess,mahak/vitess,pivanof/vitess,mapbased/vitess,mahak/vitess,sougou/vitess,HubSpot/vitess,dumbunny/vitess,applift/vitess,dcadevil/vitess,enisoc/vitess,tirsen/vitess,mapbased/vitess,rnavarro/vitess,mapbased/vitess,applift/vitess,tirsen/vitess,vitessio/vitess,mattharden/vitess,alainjobart/vitess,mahak/vitess,vitessio/vitess,tirsen/vitess,sougou/vitess,alainjobart/vitess,davygeek/vitess,HubSpot/vitess,mapbased/vitess,tirsen/vitess,davygeek/vitess,rnavarro/vitess,dcadevil/vitess,dumbunny/vitess,rnavarro/vitess,pivanof/vitess,tirsen/vitess,enisoc/vitess,tirsen/vitess,vitessio/vitess,sougou/vitess,dumbunny/vitess,mahak/vitess,enisoc/vitess,dumbunny/vitess,mahak/vitess,rnavarro/vitess,rnavarro/vitess,enisoc/vitess,enisoc/vitess,tirsen/vitess,davygeek/vitess,mahak/vitess,tirsen/vitess,dumbunny/vitess,mapbased/vitess,tinyspeck/vitess,mattharden/vitess,pivanof/vitess,sougou/vitess,dumbunny/vitess,pivanof/vitess,mattharden/vitess,dumbunny/vitess,HubSpot/vitess,mahak/vitess,alainjobart/vitess,applift/vitess,dcadevil/vitess,pivanof/vitess,applift/vitess,dcadevil/vitess,pivanof/vitess,tinyspeck/vitess,alainjobart/vitess,HubSpot/vitess,mapbased/vitess,mattharden/vitess,applift/vitess,HubSpot/vitess,applift/vitess,pivanof/vitess,mattharden/vitess,davygeek/vitess,vitessio/vitess,mapbased/vitess,pivanof/vitess,dumbunny/vitess,HubSpot/vitess,applift/vitess,tirsen/vitess,enisoc/vitess,applift/vitess,mattharden/vitess,davygeek/vitess,pivanof/vitess,vitessio/vitess,rnavarro/vitess,mattharden/vitess,applift/vitess,mattharden/vitess,sougou/vitess,dcadevil/vitess,dumbunny/vitess,mapbased/vitess,dcadevil/vitess,enisoc/vitess,sougou/vitess,alainjobart/vitess,HubSpot/vitess,davygeek/vitess,alainjobart/vitess,mattharden/vitess,tinyspeck/vitess,vitessio/vitess,vitessio/vitess,tinyspeck/vitess,dumbunny/vitess,dcadevil/vitess,rnavarro/vitess,tinyspeck/vitess,mattharden/vitess