Potential Logic Issue in nonneg_lsq()

[hayakb95]

If you try to call nonneg_lsq() with "gram = true" and "alg = pivot" then the logic will set "alg = pivot_cache".
This will then force an error in the next logic block "throw(ArgumentError("Specified algorithm $alg not recognized."))" as "alg == :pivot_cache" doesn't appear anywhere.
I think a simple fix for this is to have the first portion set "alg = pivot" (already done) and "variant = cache".

[JeffFessler]

Would you like to submit a PR that addresses it? Ideally with a test case that confirms that the fix works and doesn't break any other tests?