Fix control characters not being sanitized from shared strings in 1.3.6

In version 1.3.6, the shared strings are not sanitized to remove any control characters that may be present in the string. This causes excel files generated using shared strings to require repairs and lose data on opening.

This was fixed for later versions in f8ef8eb, so I cherry-picked the commit, fixing a conflict in `Axlsx::SharedStringsTable.to_xml_string`. I also wrote a test to confirm the control characters are actually removed.

*Note that this PR is for version 1.3.6 of the gem.* I'm not sure how you want to want to apply this patch, possibly as a point release?