feasible solution of .sol files is always 0

**Describe the bug**
When we write an initial feasible solution in a .sol file, it always gives "optimal feasible solution - 0".

**To Reproduce**
Try any writing on a .sol file of an optimizer function.

**Expected behavior**
I expected to have the first feasible solution of my optimization problem.

**Desktop (please complete the following information):**
- Operating System, version: Windows 10
- Python version:
- Python-MIP version (we recommend you to test with the latest version): latest

**Additional context**
I think that the issue is related to the function 'save_mipstart' which seems to be by default at 0 and never changes.