text: Title: Issue with write_url when internally referencing a sheet (with spaces) in the same book
  Body: Hi,
  
  I am trying to use XlsxWriter to create a table of contents that has hyperlinks that are linked to the various sheets within the book. The sheet names have spaces in them, so I follow the syntax guidelines of single-quoting the sheet:
  
  ```
  import xlsxwriter
  
  workbook = xlsxwriter.Workbook('hello.xlsx')
  worksheet = workbook.add_worksheet()
  
  call = "internal:'Summary Figure 1'!A1"
  worksheet.write_url(5, 0, call, url_format, string = string + str(end_year))
  
  ```
  When I print the string 'call', I get "internal:'Summary Figure 1'!A1" out, but when I run the program and edit the hyperlink in the output xlsx file, it shows:
  
  %22internal:'Summary%20Figure%201'!A1%22 
  
  which breaks the link.
  
  I am using Python version 3.6 and XlsxWriter 1.0.4 and Excel version 2013.
  
  I believe this same problem was fixed for external references (https://github.com/jmcnamara/XlsxWriter/issues/350), but not for internal.
  
  Thanks
  