Invalid EPub export for Kobo devices when title contains '+'

[egilli]

Issue details

When exporting an entry with a title containing the + character to epub, the exported file does not work in Kobo devices.
Other characters may trigger the problem, but this is untested (titles with : have some problems with another library).

Environment

• wallabag version (or git revision) that exhibits the issue: git rev a707643
• How did you install wallabag? git clone
• Last wallabag version that did not exhibit the issue (if applicable): N/A
• php version: PHP 7.0.10
• OS: Debian testing
• type of hosting (shared or dedicated): dedicated
• which storage system you choose at install (SQLite, MySQL/MariaDB or PostgreSQL): SQLite

Steps to reproduce/test case

1. Add an entry with the character + in its title, like https://en.wikipedia.org/wiki/C++.
2. Export this entry to the EPub format.
3. Copy the epub file to a Kobo device (tested with Kobo Glo HD).
4. Open the file with the Kobo device : the file opens, displaying only the first two pages : cover and table of content.
5. The content of the entry cannot be read, the device thinks it contains only two pages : the cover and the table of content. The content of the entry is ignored.

(6. Do the same steps, but first edit the entry's title to remove the + : the whole content of the file can now be displayed on the Kobo device.)

I'll do a pull request with a possible correction.

[egilli]

Thanks!