Fix keyword weights being flattened to 1

[Ppantaleo]

The problem

array_unique() is applied to the journal-wide list of keyword names before array_count_values() counts them:

$uniqueKeywords = array_unique(array_map('strtolower', array_column($keywords, 'name')));
$countKeywords = array_count_values($uniqueKeywords);

By the time the count runs every value is distinct, so each keyword gets a count of 1. Two consequences:

• Every word ends up the same size. fontSize.domain() in block.tpl gets [1, 1], a degenerate domain, so d3 maps every keyword to the same font size and the cloud carries no information.
• array_slice() keeps an arbitrary 50 keywords rather than the 50 most frequent, since arsort() has nothing to sort by.

The fix

Deduplicate per publication instead of across the whole journal, so a word's weight is the number of articles that use it. A keyword repeated inside a single publication still counts once.

Also passes preserve_keys: true to array_slice(): a purely numeric keyword ("2020") had its key reindexed and reached the template as "0".

Testing

Verified on OJS 3.5.0.5 against a journal with 879 published publications. The block now returns covid-19: 22, argentina: 17, editorial: 15, ..., matching the counts obtained directly via SQL. Before the change every entry was size: 1.

Note on #31

#31 touches the same lines but addresses a different problem — the block coming out empty when getBySymbolic() returns a flat array. It keeps the array_unique() before the count, so it does not change the weights. Happy to rebase if #31 lands first.