Jpeg encoded with RGB -instead of YCbCr- write the components index as "RGB" in ASCII to say it so

On ISO/IEC 10918-6:2013 (E), section 6.1: (http://www.itu.int/rec/T-REC-T.872-201206-I/en)

"Images encoded with three components are assumed to be RGB data encoded as YCbCr unless the image contains an APP14 marker segment as specified in 6.5.3, in which case the colour encoding is considered either RGB or YCbCr according to the application data of the APP14 marker segment"

But common jpeg libraries consider RGB too if components index are ASCII R (0x52), G (0x47) and B (0x42): 50861048

Issue #11931

test/pdfs/.gitignore

@@ -147,6 +147,7 @@
 !issue1055r.pdf
 !issue11713.pdf
 !issue1293r.pdf
+!issue11931.pdf
 !issue1655r.pdf
 !issue6541.pdf
 !issue2948.pdf

test/test_manifest.json

@@ -4367,6 +4367,12 @@
        "type": "eq",
        "forms": true
     },
+    {  "id": "issue11931",
+       "file": "pdfs/issue11931.pdf",
+       "md5": "9ea233037992e1f10280420a49e72845",
+       "rounds": 1,
+       "type": "eq"
+    },
     {  "id": "annotation-button-widget-annotations",
        "file": "pdfs/annotation-button-widget.pdf",
        "md5": "5cf23adfff84256d9cfe261bea96dade",