Tags case should be preserved

[ArthurHoaro]

X-issue from shaarli/Shaarli#1726 - quoting @cy7yz2rj:

Shaarli is case insensitive for tags but should be case preserving when importing.

Steps for reproducing:

export bookmarks to HTML file from Tools > Export Database
delete all bookmarks
import the file from step 1
all tags are now lowercase despite having correct case information in the HTML file

Firefox correctly preserves case when importing the same bookmark file.