Model.write() outputs incorrect objective row in MPS format

**Describe the bug**

When I formulate a model in python-mip to maximize an objective function and write it to a `.mps` file, the objective coefficients have the wrong sign.

**To Reproduce**

Construct the 0-1 knapsack model from the [examples](https://docs.python-mip.com/en/latest/examples.html) and write it to a `.mps` file.

```python
from mip import Model, xsum, maximize, BINARY

p = [10, 13, 18, 31, 7, 15]
w = [11, 15, 20, 35, 10, 33]
c, I = 47, range(len(w))

m = Model("knapsack")

x = [m.add_var(var_type=BINARY) for i in I]

m.objective = maximize(xsum(p[i] * x[i] for i in I))

m += xsum(w[i] * x[i] for i in I) <= c

m.write('knapsack.mps')
```

Now call `cbc` or `highs` on the resulting file.

```bash
$ cbc knapsack.mps
```

The optimal objective is (incorrectly) 0.

```txt
Welcome to the CBC MILP Solver
Version: 2.10.8
Build Date: May 7 2022

command line - cbc knapsack.mps (default strategy 1)
At line 1 NAME knapsack FREE
At line 2 ROWS
At line 5 COLUMNS
At line 12 RHS
At line 14 BOUNDS
At line 21 ENDATA
Problem knapsack has 1 rows, 6 columns and 6 elements
Coin0008I knapsack read with 0 errors
Continuous objective value is 0 - 0.00 seconds
Cgl0004I processed model has 0 rows, 0 columns (0 integer (0 of which binary)) and 0 elements
Cbc3007W No integer variables - nothing to do
Cuts at root node changed objective from 0 to -1.79769e+308
Probing was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
Gomory was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
Knapsack was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
Clique was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
MixedIntegerRounding2 was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
FlowCover was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
TwoMirCuts was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)
ZeroHalf was tried 0 times and created 0 cuts of which 0 were active after adding rounds of cuts (0.000 seconds)

Result - Optimal solution found

Objective value: 0.00000000
Enumerated nodes: 0
Total iterations: 0
Time (CPU seconds): 0.00
Time (Wallclock seconds): 0.00

Total time (CPU seconds): 0.00 (Wallclock seconds): 0.00
```

**Expected behavior**

This happens because the MPS file is minimizing the objective. The coefficients of the output file should be `-` in this case.

```mps
NAME knapsack FREE
ROWS
N OBJROW
L constr(0)
COLUMNS
var(0) OBJROW 10. constr(0) 11.
var(1) OBJROW 13. constr(0) 15.
var(2) OBJROW 18. constr(0) 20.
var(3) OBJROW 31. constr(0) 35.
var(4) OBJROW 7. constr(0) 10.
var(5) OBJROW 15. constr(0) 33.
RHS
RHS constr(0) 47.
BOUNDS
BV BOUND var(0) 1.
BV BOUND var(1) 1.
BV BOUND var(2) 1.
BV BOUND var(3) 1.
BV BOUND var(4) 1.
BV BOUND var(5) 1.
ENDATA
```

If I change their signs:

```mps
NAME knapsack FREE
ROWS
N OBJROW
L constr(0)
COLUMNS
var(0) OBJROW -10. constr(0) 11.
var(1) OBJROW -13. constr(0) 15.
var(2) OBJROW -18. constr(0) 20.
var(3) OBJROW -31. constr(0) 35.
var(4) OBJROW -7. constr(0) 10.
var(5) OBJROW -15. constr(0) 33.
RHS
RHS constr(0) 47.
BOUNDS
BV BOUND var(0) 1.
BV BOUND var(1) 1.
BV BOUND var(2) 1.
BV BOUND var(3) 1.
BV BOUND var(4) 1.
BV BOUND var(5) 1.
ENDATA
```

Then the solution is correct.

```txt
Result - Optimal solution found

Objective value: -41.00000000
Enumerated nodes: 0
Total iterations: 0
Time (CPU seconds): 0.01
Time (Wallclock seconds): 0.01

Total time (CPU seconds): 0.01 (Wallclock seconds): 0.01
```
**Desktop (please complete the following information):**
- Operating System, version: 11.6.7
- Python version: 3.7.8
- Python-MIP version (we recommend you to test with the latest version): 1.13.0