table = eso.query_main(column_filters={'instrument': 'vircam'}) is case sensitive

Hi,

It looks like there is a case-sensitive feature with eso.query_main.

The helpful example has:
table = eso.query_main(column_filters={'instrument': 'APICAM', 'filter_path': 'LUMINANCE',
'stime':'2019-04-26', 'etime':'2019-04-27'}, cache=False)

but all the instruments listed by list_instruments are lower case. I also note that 'APICAM/apicam' is not listed!

eso.list_instruments()
['fors1', 'fors2', 'sphere', 'vimos', 'omegacam', 'eris', 'hawki', 'isaac', 'naco', 'visir',
'vircam', 'apex', 'giraffe', 'uves', 'xshooter', 'espresso', 'muse', 'crires',
'kmos', 'sinfoni', 'amber', 'gravity', 'matisse', 'midi', 'pionier', 'wlgsu']

table = eso.query_main(column_filters={'instrument': 'vircam'}) returns:
WARNING: NoResultsWarning: Query returned no results [astroquery.eso.core]

table = eso.query_main(column_filters={'instrument': 'VIRCAM'}) works OK
print(len(table))
99

Start by reproducing the two eso.query_main calls and comparing them with eso.list_instruments(). Then inspect the implementation and tests for query_main and instrument filtering; done means the documented lower-case value 'vircam' returns the same results as 'VIRCAM' without breaking existing queries.

Written by the indexing model from the issue text.