FIX: Remove test that tests state of warnings before and after assert_warns
TST: Test that the warning registry is empty after assert_warns

Also minor doc change to the statement that this class was
copied from NumPy
fix to a minor bug with intercept
TST: Test that assert_warns is reset internally
DOC: Make it explicit that assert_warns clears the warning registry
Merge pull request #3752 from MechCoder/remove_testing_test

[MRG+2] FIX: Modify test that tests state of warnings before and after assert_warns
Merge pull request #3770 from dsullivan7/asgdintercept

[MRG] fix to a minor bug with intercept
fixing duplicated classifier in asgd test
Merge pull request #3773 from dsullivan7/sgdtestfix

[MRG] fixing duplicated classifier in asgd test
[MRG] Add quantile strategy to DummyRegressor. Fixes #3421.
Address review comments.
Address review comments.
Combine argument checks.
Modify y emptiness check.
Check that quantile is scalar, not its value.