Adding a .Pages attribute in PdfWriter to allow setting its MediaBox …

…or Resources fields

pdfrw/pdfwriter.py

@@ -262,6 +262,7 @@ def __init__(self, fname=None, version='1.3', compress=False, **kwargs):
                                      "on PdfWriter instance" % name)
                 setattr(self, name, value)
 
+        self.Pages = None
         self.pagearray = PdfArray()
         self.killobj = {}
 
@@ -310,14 +311,14 @@ def _get_trailer(self):
             self.make_canonical()
 
         # Create the basic object structure of the PDF file
+        pages = self.Pages or IndirectPdfDict()
+        pages.Type = PdfName.Pages,
+        pages.Count = PdfObject(len(self.pagearray)),
+        pages.Kids = self.pagearray
         trailer = PdfDict(
             Root=IndirectPdfDict(
                 Type=PdfName.Catalog,
-                Pages=IndirectPdfDict(
-                    Type=PdfName.Pages,
-                    Count=PdfObject(len(self.pagearray)),
-                    Kids=self.pagearray
-                )
+                Pages=pages,
             )
         )
         # Make all the pages point back to the page dictionary and