Ensure that the PDF header contains an actual number (PR 11463 follow-up)

While it would be nice to change the `PDFFormatVersion` property, as returned through `PDFDocumentProxy.getMetadata`, to a number (rather than a string) that would unfortunately be a breaking API change.
However, it does seem like a good idea to at least *validate* the PDF header version on the worker-thread, rather than potentially returning an arbitrary string.

test/pdfs/.gitignore

@@ -110,6 +110,7 @@
 !issue9949.pdf
 !bug1308536.pdf
 !bug1337429.pdf
+!bug1606566.pdf
 !issue5564_reduced.pdf
 !canvas.pdf
 !bug1132849.pdf

test/pdfs/bug1606566.pdf

@@ -0,0 +1,66 @@
+%âãÏÓ
+1 0 obj 
+<<
+/Pages 2 0 R
+/Type /Catalog
+>>
+endobj 
+2 0 obj 
+<<
+/Kids [3 0 R]
+/Count 1
+/Type /Pages
+>>
+endobj 
+3 0 obj 
+<<
+/Parent 2 0 R
+/MediaBox [0 0 200 50]
+/Resources 
+<<
+/Font 
+<<
+/F1 4 0 R
+>>
+>>
+/Contents 5 0 R
+/Type /Page
+>>
+endobj 
+4 0 obj 
+<<
+/BaseFont /Times-Roman
+/Subtype /Type1
+/Encoding /WinAnsiEncoding
+/Type /Font
+>>
+endobj 
+5 0 obj 
+<<
+/Length 42
+>>
+stream
+BT
+10 20 TD
+/F1 20 Tf
+(Bug 1606566) Tj
+ET
+
+endstream 
+endobj xref
+0 6
+0000000000 65535 f 
+0000000015 00000 n 
+0000000066 00000 n 
+0000000125 00000 n 
+0000000254 00000 n 
+0000000355 00000 n 
+trailer
+
+<<
+/Root 1 0 R
+/Size 6
+>>
+startxref
+449
+%%EOF

test/unit/api_spec.js

@@ -1121,6 +1121,28 @@ describe("api", function() {
         })
         .catch(done.fail);
     });
+    it("gets metadata, with missing PDF header (bug 1606566)", function(done) {
+      const loadingTask = getDocument(buildGetDocumentParams("bug1606566.pdf"));
+
+      loadingTask.promise
+        .then(function(pdfDocument) {
+          return pdfDocument.getMetadata();
+        })
+        .then(function({ info, metadata, contentDispositionFilename }) {
+          // The following are PDF.js specific, non-standard, properties.
+          expect(info["PDFFormatVersion"]).toEqual(null);
+          expect(info["IsLinearized"]).toEqual(false);
+          expect(info["IsAcroFormPresent"]).toEqual(false);
+          expect(info["IsXFAPresent"]).toEqual(false);
+          expect(info["IsCollectionPresent"]).toEqual(false);
+
+          expect(metadata).toEqual(null);
+          expect(contentDispositionFilename).toEqual(null);
+
+          loadingTask.destroy().then(done);
+        })
+        .catch(done.fail);
+    });
 
     it("gets data", function(done) {
       var promise = doc.getData();