Add ktlint test to plugin-maven

This verifies that the previous fix which checks if ktlint_code_style is
already defined in .editorconfig before overriding it works as expected
in plugin-maven.

Author: OlivierGenez

plugin-maven/src/test/java/com/diffplug/spotless/maven/kotlin/KtlintTest.java

@@ -55,6 +55,17 @@ void testReadCodeStyleFromEditorConfigFile() throws Exception {
 		checkIntellijIdeaStyle();
 	}
 
+	@Test
+	void testEditorConfigOverrideWithUnsetCodeStyleDoesNotOverrideEditorConfigCodeStyleWithDefault() throws Exception {
+		setFile(".editorconfig").toResource("kotlin/ktlint/ktlint_official/.editorconfig");
+		writePomWithKotlinSteps("<ktlint>\n" +
+				"  <editorConfigOverride>\n" +
+				"    <ktlint_test_key>true</ktlint_test_key>\n" +
+				"  </editorConfigOverride>\n" +
+				"</ktlint>");
+		checkKtlintOfficialStyle();
+	}
+
 	@Test
 	void testSetEditorConfigCanOverrideEditorConfigFile() throws Exception {
 		setFile(".editorconfig").toResource("kotlin/ktlint/intellij_idea/.editorconfig");