(REGRESSION) ssconvert spreadsheet->csv merges sheets instead of converting only the "active" sheet

When using the "Configurable text exporter" to convert a spreadsheet to .csv, all the sheets are concatenated into the output file even if no "sheet" option is given. Only the "active" sheet should be converted.

This appears to be a regression; the problem does not happen with ssconvert 1.12.28

Here's what the man page says:

   OPTIONS FOR THE CONFIGURABLE TEXT (*.txt) EXPORTER
   sheet  Name  of the workbook sheet to operate on.  You can specify sev‐
          eral sheets by repeating this option.  If  this  option  is  not
          given the active sheet (i. e. the sheet that was active when the
          file was saved) is used.

(In the example below the simple csv exporter could be used instead, but in real life the "configurable" exporter is needed in order to control the encoding).

STEPS TO REPRODUCE:

1. Download either of the attached demo spreadsheets (test.ods or test.xlsx)
2. ssconvert '--export-type=Gnumeric_stf:stf_assistant' /tmp/test.ods /tmp/out.csv

RESULTS: Data from both sheets inside the spreasheet are concatenated in the output .csv file

EXPECTED RESULTS: The output .csv should only contain data from the sheet which was "active" when the spreadsheet was saved.

ssconvert version '1.12.43' 
Libre Office (used to create the spreadsheet) 5.4.7.2
Ubuntu 18.10

test.xlsx

test.ods