text: Title: write doesn't call write_array_formula
  Body: Hi,
  
  I am using XlsxWriter to write array formulas. The [documentation](http://xlsxwriter.readthedocs.io/worksheet.html?#write_array_formula) claims write() should call write_array_formula() for use when it returns a single value, but it is only called by write_formula().
  
  I am using Python version 3.6 and XlsxWriter 0.9.6 and Excel version 2010 and 2016.
  
  Here is some code that demonstrates the problem:
  
  ```python
  
  import xlsxwriter
  
  # Create a new workbook and add a worksheet
  workbook = xlsxwriter.Workbook('array_formula.xlsx')
  worksheet = workbook.add_worksheet()
  
  # Write some test data.
  worksheet.write('B1', 500)
  worksheet.write('B2', 10)
  worksheet.write('C1', 300)
  worksheet.write('C2', 15)
  
  
  # Write an array formula that returns a single value
  worksheet.write_formula('A1', '{=SUM(B1:C1*B2:C2)}')
  worksheet.write('A2', '{=SUM(B1:C1*B2:C2)}')
  workbook.close()
  
  ```
  
  A1 is an array formula while A2 is a string.