Named Styles contain wrong xfId

Created originally on Bitbucket by Stephan Richter

After loading a workbook, all named styles are available. But none of the styles gets applied correctly. This is due to the fact that the parser code assigns a new fxId based on the index of the named styles instead of honoring the fxId that is stored in the XML.

The bug can be found in openpyxl.styles.named_styles, line 288, where is says:

            style._set_index(len(styles)) # assign xfId

Instead it should be:

            style._set_index(ns.xfId) # assign xfId

Unfortunately I do not know enough about hg to make a quick PR. 😞