rbm should do deep merge of options hashes
In templates, when using the c() or pc() functions, it is possible to give as 2nd or 3rd argument an hash table containing variables to be overridden. To do that, we merge this hash table into the main options hash table. However we only merge one level, which means that if the new options hash table contains variables in a second level with something like var => { option_1 => 'value1' }, then var is replaced instead of merged, and all var/* options other than option_1 are removed.
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