Fix Requests with asins and list ordering

This pull request fixes requests with asins (i.e. GetLowestOfferListingsForASIN) and the usage of lists.

The word "ASIN" ned to be capitalised in requests.
```ruby
mws.products.call(:get_lowest_offer_listings_for_ASIN, params=params)
```
...resolves now to "GetLowestOfferListingsForASIN" and not "GetLowestOfferListingsForAsin".

---

The active record Hash#to_query broke the natural-byte order of the parameters.
```ruby
[ASINList.ASIN.10, ASINList.ASIN.12, ASINList.ASIN.1]
```
...gets not correctly reordered to: 
```ruby
[ ASINList.ASIN.1, ASINList.ASIN.10, ASINList.ASIN.12] 
```