fixed erddap_tab_GET fxn to honor disk$overwrite=T

The "disk(overwrite=T)" currently  is not honored in the erddap_table fxn. So if a user changes his mind about a table saved to disk, he has to delete the file before getting new data. For example:

# initial table
cruises <- erddap_table(x='erdCalCOFIeggcntpos', fields=c('cruise'),
                        orderby='cruise', distinct=TRUE)
cruises
# oops, I forgot I wanted cruise and ship, default is overwrite=T
cruises <- erddap_table(x='erdCalCOFIeggcntpos', fields=c('cruise', 'ship'),
                        orderby=c('cruise', 'ship'), distinct=TRUE)
# nope, still reading the existing file instead of overwriting
cruises