title: Writing pdf with pypdf2 gives error
body: <p>I'm trying to write a simple script to merge two PDFs but have run into an issue when trying to save the output to disk. My code is</p>
  
  <pre><code>from PyPDF2 import PdfFileWriter, PdfFileReader
  import tkinter as tk
  from tkinter import filedialog    
  
  ### Prompt the user for the 2 files to use via GUI ###
  root = tk.Tk()
  root.update()
  file_path1 = tk.filedialog.askopenfilename(
              filetypes=[("PDF files", "*.pdf")],
              )
  
  file_path2 = tk.filedialog.askopenfilename(
              filetypes=[("PDF files", "*.pdf")],
              )
  
  ###Function to combine PDFs###
  output = PdfFileWriter()
  
  def append_pdf_2_output(file_handler):
      for page in range(file_handler.numPages):
          output.addPage(file_handler.getPage(page))
  
  #Actually combine the 2 PDFs###
  append_pdf_2_output(PdfFileReader(open(file_path1, "rb")))
  append_pdf_2_output(PdfFileReader(open(file_path2, "rb")))
  
  ###Prompt the user for the file save###
  output_name = tk.filedialog.asksaveasfile(
              defaultextension='pdf')
  
  ###Write the output to disk###
  output.write(output_name)
  output.close
  </code></pre>
  
  <p>The problem is that I get an error of </p>
  
  <p>UserWarning: File  to write to is not in binary mode. It may not be written to correctly. [pdf.py:453] Traceback (most recent call last): File "Combine2Pdfs.py", line 44, in  output.write(output_name) File "/Library/Frameworks/Python.framework/Versions/3.5/lib/pytho‌​n3.5/site-packages/P‌​yPDF2/pdf.py", line 487, in write stream.write(self.<em>header + b</em>("\n")) TypeError: write() argument must be str, not bytes</p>
  
  <p>Where have I gone wrong?</p>