Fix jp2 and hdr plugin json definition

and add a check that the json files are well-formed

i.e. they have the same number of keys than of mimetypes

src/imageformats/hdr.json

@@ -1,4 +1,4 @@
 {
-    "Keys": [ "hdr" ],
+    "Keys": [ "hdr", "hdr" ],
     "MimeTypes": [ "image/x-hdr", "image/vnd.radiance" ]
 }

src/imageformats/jp2.json

@@ -1,4 +1,4 @@
 {
     "Keys": [ "jp2", "j2k", "jpf" ],
-    "MimeTypes": [ "image/jp2" ]
+    "MimeTypes": [ "image/jp2", "image/x-jp2-codestream", "image/jpx" ]
 }