Fixes for Fields in get_params

[gamekid]

• correctly handle multiple Fields collections
• only add Fields to get_params once

When including multiple entries in the fields parameter, the query builder was not adding an ampersand between each entry. Additionally, it was adding each entry to the query twice, once in the base PathBuilder class and again in the PathCollectionBuilder class.

[pablorsk]

Thanks @gamekid !

2.1.8 relased! :tada:

[gamekid]

Thanks for getting this out!