Escape backslashes upon export for better import

[JasonH09]

Exporting a WordPress site and importing it again will lose backslashes in Post content.
For example, if you had a post discussing c:\Windows\System32\calc.exe it would be imported as c:WindowsSystem32calc.exe

[rmccue]

I believe this is a bug in the importer, not the exporter. The exporter is writing the data out correctly (as regular text), but the importer is removing a layer of slashes.

(See also ​this issue on the v2 beta importer)

[mcbsys]

(Partially) fixed in Importer version 0.6.3. See:

https://core.trac.wordpress.org/ticket/21007
​https://github.com/boonebgorges/wordpress-importer/commit/998f7fc75c6916355859b7e791e60b97a3efccdb

Backslashes are still stripped from comments.