Check if file is PDF by magic number. Closes documentcloud#98

See http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/pdf_reference_1-7.pdf page 92

Author: tmaier

lib/docsplit/transparent_pdfs.rb

@@ -9,7 +9,7 @@ module TransparentPDFs
     def ensure_pdfs(docs)
       [docs].flatten.map do |doc|
         ext = File.extname(doc)
-        if ext.downcase == '.pdf'
+        if ext.downcase == '.pdf' || File.open(doc, &:readline) =~ /\A\%PDF-\d+(\.\d+)?$/
           doc
         else
           tempdir = File.join(Dir.tmpdir, 'docsplit')
@@ -23,4 +23,4 @@ def ensure_pdfs(docs)
 
   extend TransparentPDFs
 
-end
+end