Use Hash#fetch in Api::Response params

This will raise errors on missing keys
Move Api::TaxAmount testing out of Api::Response
Remove redundant RequestSpecHelper include
Make Api::Response tests read better
Remove redundant RequestSpecHelper and spacing from test
Use suretax_wrap_response instead of hardcoded string
Create objects for all parts of the API response

- Added Group for 'GroupList', accessible via Api::Response#groups.
- Added Tax for 'TaxList', accessible via Api::Group#taxes.
- Currency amounts in Tax are TaxAmount instances.