Rewrote LicenseHeaderStep in terms of FormatterStep.Strict.

Note that in the case where the license header and delimiter are supplied as strings, it makes sense to store the state eagerly, whereas when the license header is supplied in a file, we load that file lazily, only if we actually need to.

src/main/java/com/diffplug/gradle/spotless/FormatExtension.java

@@ -263,7 +263,9 @@ public void indentWithTabs() {
 	 *            Spotless will look for a line that starts with this to know what the "top" is.
 	 */
 	public void licenseHeader(String licenseHeader, String delimiter) {
-		customLazy(LicenseHeaderStep.NAME, () -> new LicenseHeaderStep(licenseHeader, delimiter)::format);
+		steps.add(FormatterStep.create(LicenseHeaderStep.NAME,
+				new LicenseHeaderStep(licenseHeader, delimiter),
+				LicenseHeaderStep::format));
 	}
 
 	/**
@@ -273,7 +275,9 @@ public void licenseHeader(String licenseHeader, String delimiter) {
 	 *            Spotless will look for a line that starts with this to know what the "top" is.
 	 */
 	public void licenseHeaderFile(Object licenseHeaderFile, String delimiter) {
-		customLazy(LicenseHeaderStep.NAME, () -> new LicenseHeaderStep(getProject().file(licenseHeaderFile), getEncoding(), delimiter)::format);
+		steps.add(FormatterStep.createLazy(LicenseHeaderStep.NAME,
+				() -> new LicenseHeaderStep(getProject().file(licenseHeaderFile), getEncoding(), delimiter),
+				LicenseHeaderStep::format));
 	}
 
 	/** Sets up a format task according to the values in this extension. */

src/main/java/com/diffplug/gradle/spotless/LicenseHeaderStep.java

@@ -17,6 +17,7 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.io.Serializable;
 import java.nio.charset.Charset;
 import java.nio.charset.StandardCharsets;
 import java.nio.file.Files;
@@ -26,7 +27,9 @@
 import org.gradle.api.GradleException;
 
 /** Prefixes a license header before the package statement. */
-public class LicenseHeaderStep {
+public class LicenseHeaderStep implements Serializable {
+	private static final long serialVersionUID = 1L;
+
 	public static final String NAME = "LicenseHeader";
 
 	private final String license;