Configuration improvements

This PR makes Connection and Api::Request configuration-aware.

## Configuration
- Removes 'default' from attribute names

## Connection
- Uses the values in Suretax.configuration by default
- Takes a Configuration object override as an optional argument

## Api::Request
- Uses the values in Suretax.configuration by default
- Takes a Configuration object override as an optional argument
- Allows you to override configuration by specifying values passed as arguments.