[FreshRSS] Case insensitive charsets

Ensure charsets are compared without case sensitivity to avoid
duplicates. strtoupper() was already used in one place, but was lacking
in two other places.
Merge pull request #352 from Alkarex/strtoupper_charset

[FreshRSS] Case insensitive charsets
Merge branch 'master' into always-order-by-date