fix: PDF servlet

Since the update of `bastik` the PDF servlet didn't work anymore.

Problems:
- No PDF UnitTest existed
- Pom dependency `org.apache.xmlgraphics.batik-all` change nearly every dependency to `optional` starting with version `1.15`, hence some important dependencies like the SVG converter were missing
- `DiagramResponse.CONTENT_TYPE` had no mime type value for PDF

Changes:
- add PDF UnitTest
- remove `batik-all` dependency and only include the dependencies PlantUML requires for the PDF generation:
  * batik-dom
  * batik-svgrasterizer (includes batik-dom)
  * batik-svggen
  * fop
- remove own `DiagramResponse.CONTENT_TYPE` mapping and use `FileFormat.getMimeType()`

src/test/java/net/sourceforge/plantuml/servlet/TestPDF.java

@@ -0,0 +1,38 @@
+package net.sourceforge.plantuml.servlet;
+
+import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.URL;
+
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.Test;
+
+import net.sourceforge.plantuml.servlet.utils.TestUtils;
+import net.sourceforge.plantuml.servlet.utils.WebappTestCase;
+
+
+public class TestPDF extends WebappTestCase {
+
+    /**
+     * Verifies the generation of the PDF for the Bob -> Alice sample
+     */
+    @Test
+    public void testSimpleSequenceDiagram() throws IOException {
+        final URL url = new URL(getServerUrl() + "/pdf/" + TestUtils.SEQBOB);
+        final HttpURLConnection conn = (HttpURLConnection)url.openConnection();
+        // Analyze response
+        // Verifies HTTP status code and the Content-Type
+        Assertions.assertEquals(200, conn.getResponseCode(), "Bad HTTP status received");
+        Assertions.assertEquals(
+            "application/pdf",
+            conn.getContentType().toLowerCase(),
+            "Response content type is not PDF"
+        );
+        // Get the content and verify its size
+        String diagram = getContentText(conn);
+        int diagramLen = diagram.length();
+        Assertions.assertTrue(diagramLen > 1500);
+        Assertions.assertTrue(diagramLen < 2000);
+    }
+
+}