Only remove the unescape prefix character when parsing

[jgmchan]

This PR addresses

• Currently the unescape_formulas option when parsing will remove any first character of a cell as long as the second character is part of the @escape_formula_start. This causes issues when the data is something like this:

  "A-1","B+2","C=3","'@D"

  which should parse to this:

  ["A-1", "B+2", "C=3", "@D"]

  but parses to this instead:

  ["-1", "+2", "=3", "@D"]

  This change will check that the first character is actually the escape character we want before removing it.

• Slight fix up of the documentation

[jgmchan]

The failing tests doesn't seem to be related to my change and I can't figure out how to kick it off again. Any ideas @beatrichartz ?

[beatrichartz]

Looking good, merged. Thanks!

[beatrichartz]

Published in 3.2.2