Unexpected behavior at certain radius values for astroquery.imcce.Skybot.cone_search

When I use this set of coordinates at this time I get an empty result for one radius:
```
from astroquery.imcce import Skybot
from astropy.time import Time
from astropy.coordinates import SkyCoord
import astropy.units as u
epoch = Time('55362.69789455', format='mjd')
field = SkyCoord(180.16294575*u.deg, 0.04415185*u.deg)
Skybot.cone_search(field, 19.789250330604407*u.arcmin, epoch, get_raw_response=True, find_asteroids=True)
```
result: # Flag: -1\n# Ticket: 181292695259995135\nSkyBoT comet conesearch -> MySQL server (asterws3.imcce.fr): \n

But a non-empty result for the following marginally smaller radius:

`Skybot.cone_search(field, 19.78925033060440*u.arcmin, epoch, get_raw_response=True, find_asteroids=True)`

result: # Flag: 1\n# Ticket: 181292699247551035\n# Num | Name | RA(h) | DE(deg) | Class | Mv | Err(arcsec) | d(arcsec) | dRA(arcsec/h) | dDEC(arcsec/h) | Dg(ua) | Dh(ua) | Phase(deg) | SunElong(deg) | x(au) | y(au) | z(au) | vx(au/d) | vy(au/d) | vz(au/d) | Ref. Epoch(JD) \n 397879 | 2008 UC100 | 12 00 31.7111 | +00 03 45.841 | MB>Inner | 23.4 | 2.017 | 129.546 | 22.1342 | -9.885 | 2.40648073312 | 2.70744509418 | 21.95 | 95.76 | -2.518461043 | -0.907950204 | -0.388408409 | 0.003630443 | -0.007884952 | -0.003471052 | 2455360 \n ... etc.

There seems to be no correlation with number of decimal places or with the size of the radius value in determining what breaks the code. This may be an issue with SkyBot itself, I am not sure.

Start with the astroquery.imcce.Skybot.cone_search entry point and reproduce the supplied request using the two nearly identical radii and get_raw_response=True. Compare the returned SkyBoT responses to determine whether the discrepancy originates in the client or the external service; done means the behavior is explained and, if client-side, covered by a regression test.

Written by the indexing model from the issue text.