title: Python openpyxl: Read only mode returns a different row count
body: <p>I'm working with an xlsx file using openpyxl, but when I open this xlsx file in read_only mode, I can only seem to access the first 10 rows.  I don't see this with all xlsx files. Is there something about the data in an xlsx file that would trigger the StopIteration early with read_only as True but not when False?  All data in the spreadsheet is (theoretically) uniform and plain text. </p>
  
  <p>This works as expected for book2, but not for book1 as shown below.</p>
  
  <pre><code>&gt;&gt;&gt; import openpyxl
  &gt;&gt;&gt; a = openpyxl.load_workbook("book1.xlsx", read_only=True)
  &gt;&gt;&gt; a.active.max_row
  10
  &gt;&gt;&gt; a = openpyxl.load_workbook("book1.xlsx", read_only=False)
  &gt;&gt;&gt; a.active.max_row
  20082
  
  &gt;&gt;&gt; a = openpyxl.load_workbook("book2.xlsx", read_only=True)
  &gt;&gt;&gt; a.active.max_row
  1069
  &gt;&gt;&gt; a = openpyxl.load_workbook("book2.xlsx", read_only=False)
  &gt;&gt;&gt; a.active.max_row
  1069
  </code></pre>