ENH: optimize: curve_fit now warns if pcov is undetermined

Marked the test as a knownfailure for now. The test fails because
assert_warns in numpy.testing should clean __warningregistry__...
FIX: re-enable test for indertemitate covariance warning

Fixed the bogus waring in test_array_like and thus now we can
re-enable the test for the issuing of OptimizeWarning from curve_fit
when the covariance matrix is indeterminate. This works as long as in
the future new tests don't trigger the warning somewhere else in scipy
test suite ;)
PEP8: fix line too long
ENH: mention OptimizeWarning in curve_fit docstring
Merge pull request #3930 from otizonaizit/curve_fit-warning

ENH: optimize: curve_fit now warns if pcov is undetermined