Always pass a mutable keyword arguments hash to filters

[dylanahsmith]

Problem

While looking at application test failures for #60, I noticed a few FrozenError: can't modify frozen Hash exceptions in one of the filters. The filter was modifying the options hash that was passed to it.

Actually, the rb_hash_freeze(const_keyword_args); line in that PR (https://github.com/Shopify/liquid-c/pull/60/files#r504912592) is missing on master, but it is passing the same hash to filter call. That could result in even more subtle problems. As mentioned on the common for that line

I think we are going to have to always freeze or always dup the hash. Otherwise, it is too easy to test a filter using dynamic keyword arguments and then get errors in production when a frozen hash is passed in for constant keyword arguments.

Solution

Most filters take positional arguments, so it might not be worth trying to optimize for constant keyword arguments at the moment. So I've just removed that special case to make the code simpler, by always building a new hash for filter keyword arguments.