Only remove the unescape prefix character when parsing

* 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

lib/csv/decoding/decoder.ex

@@ -37,7 +37,7 @@ defmodule CSV.Decoding.Decoder do
   * `:validate_row_length` – When set to `true`, will take the first row of
       the csv or its headers and validate that following rows are of the same
       length. Defaults to `false`.
-  * `:escape_formulas`      – When set to `true`, will remove formula escaping
+  * `:unescape_formulas`   – When set to `true`, will remove formula escaping
       inserted to prevent [CSV Injection](https://owasp.org/www-community/attacks/CSV_Injection).
 
   ## Examples

lib/csv/decoding/parser.ex

@@ -88,11 +88,14 @@ defmodule CSV.Decoding.Parser do
     unescape_formulas = options |> Keyword.get(:unescape_formulas, @unescape_formulas)
 
     if unescape_formulas do
-      formula_pattern = :binary.compile_pattern(@escape_formula_start)
+      formula_pattern =
+        @escape_formula_start
+        |> Enum.map(fn char -> @escape_formula_prefix <> char end)
+        |> :binary.compile_pattern()
 
       fn field ->
         case :binary.match(field, formula_pattern) do
-          {1, _} -> binary_part(field, 1, byte_size(field) - 1)
+          {0, _} -> binary_part(field, 1, byte_size(field) - 1)
           _ -> field
         end
       end

lib/csv/defaults.ex

@@ -18,6 +18,7 @@ defmodule CSV.Defaults do
       @escape_formulas false
       @unescape_formulas false
       @escape_formula_start ["=", "-", "+", "@"]
+      @escape_formula_prefix "'"
     end
   end
 end

lib/csv/encoding/encode.ex

@@ -47,7 +47,7 @@ defimpl CSV.Encode, for: BitString do
 
     data =
       if escape_formulas and String.starts_with?(data, @escape_formula_start) do
-        "'" <> data
+        @escape_formula_prefix <> data
       else
         data
       end

test/decoding/parser_test.exs

@@ -151,14 +151,25 @@ defmodule DecodingTests.ParserTest do
   end
 
   test "removes escaping for formula when unescape_formulas is set to true" do
-    input = [["=1+1", ~S(=1+2";=1+2), ~S(=1+2'" ;,=1+2)], ["-10+7"], ["+10+7"], ["@A1:A10"]]
+    input = [
+      ["=1+1", ~S(=1+2";=1+2), ~S(=1+2'" ;,=1+2)],
+      ["-10+7"],
+      ["+10+7"],
+      ["@A1:A10"],
+      ["X-1"],
+      ["B+1"],
+      ["C=1"]
+    ]
 
     assert encode_decode_loop([input], escape_formulas: true, unescape_formulas: true) == [
              ok: [
                "=1+1=1+2\";=1+2=1+2'\" ;,=1+2",
                "-10+7",
                "+10+7",
-               "@A1:A10"
+               "@A1:A10",
+               "X-1",
+               "B+1",
+               "C=1"
              ]
            ]
   end