Extend Strainer with filters in a predictable order

Currently Strainer saves the registered filter modules in a hash, keyed by the name of the module.

As Ruby 1.8 doesn't have sorted Hashes, the final order in which the modules are included into the class is unpredictable. The hashing is also unnecessary as the key is never used.

This patch saves the filter list in an array which leads to the extension in the order in which the modules are registered which allows later modules to reliably overwrite existing filter methods.