Add test to make sure parser can detect explicit interface implementations

Author: Per Gårdebrink

tests/CommandLine.Tests/Unit/Issue591ests.cs

@@ -0,0 +1,29 @@
+using System.Linq;
+using CommandLine.Tests.Fakes;
+using CommandLine.Text;
+using FluentAssertions;
+using Xunit;
+using Xunit.Abstractions;
+
+//Issue #591
+//When options class is only having explicit interface declarations, it should be detected as mutable.
+
+namespace CommandLine.Tests.Unit
+{
+    public class Issue591ests
+    {
+        [Fact]
+        public void Parse_option_with_only_explicit_interface_implementation()
+        {
+            string actual = string.Empty;
+
+            var arguments = new[] { "--inputfile", "file2.txt" };
+            var result = Parser.Default.ParseArguments<Options_With_Only_Explicit_Interface>(arguments);
+            result.WithParsed(options => {
+                actual = ((IInterface_With_Two_Scalar_Options)options).InputFile;
+            });
+
+            actual.Should().Be("file2.txt");
+        }
+    }
+}