PDF metadata missing if \header is within a \bookpart block

[Federico Bruni]

If you compile the following snippet, you'll see no title metadata in 
the PDF:

\version "2.19.43"

\bookpart {
 \header {
   title = "Title"
   pdftitle = "PDF title"
 }
 { c1 }
}

This happens because \header is within a \bookpart block. If you move 
it outside \bookpart, the title appear in PDF metadata.

It's a bug? I think so. LilyPond should check if there's a \header 
block in an higher level (\book); if there is no \header in a higher 
level, it should use the \header values within \bookpart.

This is important to me because:

1. Most of my songs are wrapped in \bookpart blocks, because I can then 
make a book just including the files from a master \book file
2. As a result of this bug, all my single songs in my LilyPond score 
collection are missing PDF metadata (so they appear as "untitled" in 
Gnome Documents, the application I use to quickly find my local PDF)