Impossible to save nested S-Groups in Extended SMILES and SMARTS formats

**Steps to reproduce:**

1. Open attached Combination_Sgroups.molfile in Ketcher. Combination_Sgroups.mol
[Combination_Sgroups.zip](https://github.com/epam/ketcher/files/5182011/Combination_Sgroups.zip)
2. Click the Save As button. Choose the 'Extended SMILES' format.

**Actual Result:**
Error message appears: Convert error! IndigoException: SMILES saver: overlapping (nested?) repeating units can not be saved.
The developer's console contains the message: /v2/indigo/convert 400 (BAD REQUEST)

**Expected Result**

User should be able to save nested S-Groups in Extended SMILES and SMARTS formats

Note:
The same situation can be observed while saving in SMARTS format