Only include zip files when using spring jar

Update JarCommand to only include nested libraries that are actually
zip files. Similar to commit 38585bf3 which introduced the same
functionality to the Repackager.

Fixes gh-2094

spring-boot-cli/src/main/java/org/springframework/boot/cli/command/jar/JarCommand.java

@@ -74,6 +74,8 @@ public class JarCommand extends OptionParsingCommand {
 
 	private static final Layout LAYOUT = new Layouts.Jar();
 
+	private static final byte[] ZIP_FILE_HEADER = new byte[] { 'P', 'K', 3, 4 };
+
 	public JarCommand() {
 		super("jar", "Create a self-contained "
 				+ "executable jar file from a Spring Groovy script",
@@ -252,12 +254,36 @@ public class JarCommand extends OptionParsingCommand {
 
 		private void addDependency(JarWriter writer, File dependency)
 				throws FileNotFoundException, IOException {
-			if (dependency.isFile()) {
+			if (dependency.isFile() && isZip(dependency)) {
 				writer.writeNestedLibrary("lib/", new Library(dependency,
 						LibraryScope.COMPILE));
 			}
 		}
 
+		private boolean isZip(File file) {
+			try {
+				FileInputStream fileInputStream = new FileInputStream(file);
+				try {
+					return isZip(fileInputStream);
+				}
+				finally {
+					fileInputStream.close();
+				}
+			}
+			catch (IOException ex) {
+				return false;
+			}
+		}
+
+		private boolean isZip(InputStream inputStream) throws IOException {
+			for (int i = 0; i < ZIP_FILE_HEADER.length; i++) {
+				if (inputStream.read() != ZIP_FILE_HEADER[i]) {
+					return false;
+				}
+			}
+			return true;
+		}
+
 	}
 
 	/**