Fix originGeometry value generation

This should fix #979
diff --git a/coders/json.c b/coders/json.c
index ef8d9a8..eda1206 100644
--- a/coders/json.c
+++ b/coders/json.c
@@ -1449,7 +1449,7 @@
       (double) image->page.width,(double) image->page.height,
       (double) image->page.x,(double) image->page.y);
   if ((image->page.x != 0) || (image->page.y != 0))
-    (void) FormatLocaleFile(file,"    \"originGeometry\": %+.20g%+.20g,\n",
+    (void) FormatLocaleFile(file,"    \"originGeometry\": \"%+.20g%+.20g\",\n",
       (double) image->page.x,(double) image->page.y);
   JSONFormatLocaleFile(file,"    \"dispose\": %s,\n",
     CommandOptionToMnemonic(MagickDisposeOptions,(ssize_t) image->dispose));