Unescape escaped unicode characters

Excel for Windows escapes `\r` into `_x000D_` because it is not permitted in an XML 1.0 document.

I did this fix in `RubyXL::Text`, but other class like `ST_Xstring` is more appropriate, I think.
If this is not good, please tell me it.

> *22.9.2.19 ST_Xstring (Escaped String) String of characters with
> support for escaped invalid-XML characters. For all characters which
> cannot be represented in XML as defined by the XML 1.0 specification,
> the characters are escaped using the Unicode numerical character
> representation escape character format _xHHHH_,  where H represents a
> hexadecimal character in the character's value. [Example: The Unicode
> character 8 is not permitted in an XML 1.0 document,  so it must be
> escaped as _x0008_. end example]*