Include charset in text/html responses (#8777) (#9123)

Addresses #8775 for version 8.1+

server/src/main/java/com/vaadin/server/BootstrapHandler.java

 
     private void writeBootstrapPage(VaadinResponse response, String html)
             throws IOException {
-        response.setContentType("text/html");
+        response.setContentType(
+                ApplicationConstants.CONTENT_TYPE_TEXT_HTML_UTF_8);
         try (BufferedWriter writer = new BufferedWriter(
                 new OutputStreamWriter(response.getOutputStream(), "UTF-8"))) {
             writer.append(html);
         document.child(0).before(doctype);
 
         Element head = document.head();
-        head.appendElement("meta").attr("http-equiv", "Content-Type")
-                .attr("content", "text/html; charset=utf-8");
+        head.appendElement("meta").attr("http-equiv", "Content-Type").attr(
+                "content", ApplicationConstants.CONTENT_TYPE_TEXT_HTML_UTF_8);
 
         /*
          * Enable Chrome Frame in all versions of IE if installed.

server/src/main/java/com/vaadin/server/VaadinServlet.java

 import com.vaadin.annotations.VaadinServletConfiguration.InitParameterName;
 import com.vaadin.sass.internal.ScssStylesheet;
 import com.vaadin.server.communication.ServletUIInitHandler;
+import com.vaadin.shared.ApplicationConstants;
 import com.vaadin.shared.JsonConstants;
 import com.vaadin.shared.Version;
 import com.vaadin.ui.UI;
                 output += "</a>";
             }
             getService().writeStringResponse(response,
-                    "text/html; charset=UTF-8", output);
+                    ApplicationConstants.CONTENT_TYPE_TEXT_HTML_UTF_8, output);
         }
     }
 

server/src/main/java/com/vaadin/server/communication/FileUploadHandler.java

 import com.vaadin.server.VaadinRequest;
 import com.vaadin.server.VaadinResponse;
 import com.vaadin.server.VaadinSession;
+import com.vaadin.shared.ApplicationConstants;
 import com.vaadin.ui.UI;
 import com.vaadin.ui.Upload.FailedEvent;
 
      */
     protected void sendUploadResponse(VaadinRequest request,
             VaadinResponse response) throws IOException {
-        response.setContentType("text/html");
+        response.setContentType(
+                ApplicationConstants.CONTENT_TYPE_TEXT_HTML_UTF_8);
         try (OutputStream out = response.getOutputStream()) {
             final PrintWriter outWriter = new PrintWriter(
                     new BufferedWriter(new OutputStreamWriter(out, "UTF-8")));

server/src/main/java/com/vaadin/ui/LoginForm.java

 import java.util.Map;
 
 import com.vaadin.server.StreamResource;
+import com.vaadin.shared.ApplicationConstants;
 import com.vaadin.shared.Registration;
 import com.vaadin.shared.ui.loginform.LoginFormConstants;
 import com.vaadin.shared.ui.loginform.LoginFormRpc;
 
         StreamResource resource = new StreamResource(new LoginStreamSource(),
                 LoginFormConstants.LOGIN_RESOURCE_NAME);
-        resource.setMIMEType("text/html; charset=utf-8");
+        resource.setMIMEType(ApplicationConstants.CONTENT_TYPE_TEXT_HTML_UTF_8);
         resource.setCacheTime(-1);
         setResource(LoginFormConstants.LOGIN_RESOURCE_NAME, resource);
 

shared/src/main/java/com/vaadin/shared/ApplicationConstants.java

      */
     public static final String WIDGETSET_VERSION_ID = "wsver";
 
+    /**
+     * Content type to use for text/html responses (should always be UTF-8).
+     */
+    public static final String CONTENT_TYPE_TEXT_HTML_UTF_8 = "text/html; charset=utf-8";
+
 }