Dictionary::extend does not merge sub dictionaries

[drahnr]

Attempting to impl DCTDecode support uncovered an issue with Dictionary::extend which does not merge sub dictionaries recursively as anticipated.

[raimundomartins]

I also expected it to merge the 2 dictionaries. But when extending these two:

<</Type /Page/Resources [1 0 R 2 0 R]/XObject <</X1 1 0 R>>>>
<</Type /Page/Resources 3 0 R/XObject <</X2 2 0 R>>>>

I obtain
<</Type /Page/Resources [3 0 R [1 0 R 2 0 R]]/XObject <</X2 2 0 R>>>>
when I expected
<</Type /Page/Resources [3 0 R 1 0 R 2 0 R]/XObject <</X2 2 0 R/X1 1 0 R>>>>
Besides the missing X1, the array is not properly extended. This has an impact on the example PDF merging code if merging 3 PDFs. Notice that if I swap the merging order, I instead obtain:
<</Type /Pages/Resources [3 0 R 1 0 R 2 0 R]/XObject <</X1 1 0 R>>>>
The array now merges correctly, but X2 is still missing.

EDIT: Actually, I implemented code that does what I expected and was surprised (for 1sec) to see the outcome was
<</Type [/Pages /Pages]/Resources [1 0 R 2 0 R 3 0 R]/XObject <</X1 1 0 R/X2 2 0 R>>>>
So this might not be such a trivial matter after all. Maybe all I need is to check that the value is the same and not make an array in such a case, but it could have other consequences.