Add test verifying that protoc accepts trailing backslashes on Windows, which…

Add test verifying that protoc accepts trailing backslashes on Windows, which some users claim doesn't work.  Seems to work on my machine.

src/google/protobuf/compiler/command_line_interface_unittest.cc

@@ -539,6 +539,24 @@ TEST_F(CommandLineInterfaceTest, WindowsOutputPathAndParameter) {
   EXPECT_EQ("bar", generator->parameter_);
 }
 
+TEST_F(CommandLineInterfaceTest, TrailingBackslash) {
+  // Test that the directories can end in backslashes.  Some users claim this
+  // doesn't work on their system.
+
+  RegisterGenerator("test_generator", "--test_out",
+                    "output.test", "Test output.");
+
+  CreateTempFile("foo.proto",
+    "syntax = \"proto2\";\n"
+    "message Foo {}\n");
+
+  Run("protocol_compiler --test_out=$tmpdir\\ "
+      "--proto_path=$tmpdir\\ foo.proto");
+
+  ExpectNoErrors();
+  ExpectGenerated("test_generator", "", "foo.proto", "Foo", "output.test");
+}
+
 #endif  // defined(_WIN32) || defined(__CYGWIN__)
 
 TEST_F(CommandLineInterfaceTest, PathLookup) {