make monetized_attributes a hash_with_indifferent_access

[bsharpe]

monetized_attributes was getting created as a normal hash in one place and as a HWIA in another. By making it always a HWIA, we don't care if the accessed attr in validator is a symbol or a string.

We noticed this when running our tests -- validation was failing because @attr.to_s was being used to look up the monetized_attribute but the key was a symbol.

[LeKristapino]

No tests? :(

[bsharpe]

ok... tried to create tests... couldn't get it to fail.

Then realized it's because we had to create a completely new version of monetize that includes an exchange date (what is $100 CAD worth in USD a year ago.)

Our version of monetize didn't call track_monetized_attribute which does create a HashWithIndifferentAccess, instead relying on the other creation of the hash in monetized_attributes which does NOT create a HWIA.

So, the bug was ours after all, but I feel that both hashes should be created the same way (as a HWIA).

Also...the complexity of the monetize method made it very difficult to add an exchange_date parameter without copy/pasting the whole thing and changing just the bits we needed to.

[antstorm]

@bsharpe yeah, I agree, monetize method has grown too complex, but there's some work around refactoring it — #425.

Re this PR — I'm happy to merge it, but it needs some tests to make sure others know that this is intentional.