test_shadow.py tests fail due to non-converging dummy Shadow #663

____________ ERROR at setup of test_plotter_has_correct_attributes _____________

    @pytest.fixture()
    def dummy_data():
        mass = 1 * u.kg
        fov = 30 * u.km
>       shadow = Shadow(mass=mass, fov=fov, n_rays=1000)

tests/test_plotting/test_rays/test_shadow.py:15: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
/usr/lib/python3/dist-packages/astropy/units/decorators.py:313: in wrapper
    return_ = wrapped_function(*func_args, **func_kwargs)
einsteinpy/rays/shadow.py:29: in __init__
    root = newton(self._root_equation, 0.1, args=(i,))
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

func = <bound method Shadow._root_equation of <einsteinpy.rays.shadow.Shadow object at 0x7fa325ad9670>>
x0 = 0.1, fprime = None, args = (5.196152422706632,), tol = 1.48e-08
maxiter = 50, fprime2 = None, x1 = None, rtol = 0.0, full_output = False
disp = True

    def newton(func, x0, fprime=None, args=(), tol=1.48e-8, maxiter=50,
               fprime2=None, x1=None, rtol=0.0,
               full_output=False, disp=True):
[…]    
        if disp:
            msg = ("Failed to converge after %d iterations, value is %s."
                   % (itr + 1, p))
>           raise RuntimeError(msg)
E           RuntimeError: Failed to converge after 50 iterations, value is nan.

/usr/lib/python3/dist-packages/scipy/optimize/_zeros_py.py:391: RuntimeError