Remove redundant invocations of .toString()

Change-Id: I80b73b653e97904605dc62484a7448f3bfbf721c

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

@@ -80,7 +80,7 @@ public class ExternalResource implements Resource, Serializable {
             throw new RuntimeException("Source must be non-null");
         }
 
-        this.sourceURL = sourceURL.toString();
+        this.sourceURL = sourceURL;
     }
 
     /**
@@ -113,7 +113,7 @@ public class ExternalResource implements Resource, Serializable {
     @Override
     public String getMIMEType() {
         if (mimeType == null) {
-            mimeType = FileTypeResolver.getMIMEType(getURL().toString());
+            mimeType = FileTypeResolver.getMIMEType(getURL());
         }
         return mimeType;
     }

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

@@ -84,7 +84,7 @@ public class ThemeResource implements Resource {
      */
     @Override
     public String toString() {
-        return resourceID.toString();
+        return resourceID;
     }
 
     /**