Unexpected behaviour for dimod.make_quadratic

**Description**
dimod.make_quadratic has a keyword argument `strength`. According to the docs, this variable should take **positive** values. However, positive values seem to return incorrect results, while **negative** values do work.

**To Reproduce**
```
import numpy as np
import matplotlib.pyplot as pt
import dimod

def bqm_to_dict(bqm, vars):
''' convert BQM to dictionary '''
d = {}
for i,j,v in bqm.iter_quadratic():
d[(vars.index(i),vars.index(j))] = v
for i,v in bqm.iter_linear():
d[(vars.index(i),)] = v
d[()] = bqm.offset
return d

def eval_energy(merged_dict, X):
''' evaluate energy of PUSO/BUSO '''
energy = 0.
for idx, val in merged_dict.items():
energy -= val * np.prod([X[int(i)] for i in idx])
return energy

# generate random PUSO problem with 3 variables
coeffs = np.random.rand(7)*2-1
dict_puso = {
('0',): coeffs[0],
('1',): coeffs[1],
('2',): coeffs[2],
('0', '1'): coeffs[3],
('0', '2'): coeffs[4],
('1', '2'): coeffs[5],
('0', '1', '2'): coeffs[6]}

# quadratization
bqm = dimod.make_quadratic(poly=dict_puso, strength=10, vartype=dimod.SPIN)

# reorder variables
vars_head = ['0','1','2']
vars_tail = [v for v in bqm.variables if not v in vars_head]
vars = vars_head + vars_tail

# get dictionary representation of BQM
dict_buso = bqm_to_dict(bqm, vars=vars)

# visualize energy landscape over possible spin configurations
xvals, yvals_puso, yvals_buso = [], [], []
for x in [-1,1]:
for y in [-1,1]:
for z in [-1,1]:
E_puso = eval_energy(dict_puso, np.array([x,y,z]))

E_buso = np.inf
for aux1 in [-1,1]:
for aux2 in [-1,1]:
temp = eval_energy(dict_buso, np.array([x,y,z,aux1,aux2]))
if temp < E_buso:
E_buso = temp

xvals.append(f'({x},{y},{z})')
yvals_puso.append(E_puso)
yvals_buso.append(E_buso)

pt.plot(xvals, yvals_puso)
pt.title('PUSO')
pt.show()

pt.plot(xvals, yvals_buso)
pt.title('BUSO')
pt.show()
```

**Expected behavior**
The 2 plots generated by the code above should be similar (as they represent the energy landscape before and after quadratization). This isn't the case for positive values of `strength`, while it is for negative values

**Environment:**
- dimod: 0.12.14