Runtime error when upload file with special string pattern

[vietdht]

Issue

Get STRING_OFFSET_LENGTH_TOO_LARGE runtime error in ZCL_EXCEL_READER_2007~UNESCAPE_STRING_VALUE
This happens if my cell value contain string pattern like _X ... _X

Reproduce

Upload file with have cell value equals to _XX_01_XX

[sandraros]

Bug reproduced via ABAP Unit (duplicate skip_when_not_escaped and use the value _XX_01_XX).

I realize that the two methods unescape_string_value and provided_string_is_escaped deserve a little rewrite, because it's difficult to understand how they exactly deal with Unicode character entities like _x0041_ and offsets and lengths. One issue is that the abap2xlsx code is ABAP 7.02 and accepts arithmetic expressions only in assignments. The rewrite must still execute fast.

Would you mind proposing a pull request to fix the bug? Thank you.