test: add test for the default case

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---

Author: anandkaranubc

lib/node_modules/@stdlib/math/base/special/heaviside/test/test.native.js

@@ -73,6 +73,18 @@ tape( 'the function returns `1` if `x` is positive', opts, function test( t ) {
 	t.end();
 });
 
+tape( 'by default, the function returns `NaN` if provided `+-0`', opts, function test( t ) {
+	var v;
+
+	v = heaviside( -0.0 );
+	t.strictEqual( isnan( v ), true, 'returns expected value' );
+
+	v = heaviside( +0.0 );
+	t.strictEqual( isnan( v ), true, 'returns expected value' );
+
+	t.end();
+});
+
 tape( 'if the `continuity` option is `discontinuous`, the function returns `NaN` if provided `+-0`', opts, function test( t ) {
 	var v;