Task 343 internal decimal object instead of delegating

[Task 343] (https://integracredit.acunote.com/projects/44826/tasks/343) - Refactor to avoid delegating

### Changes
* Removed delegating of BigDecimal in favor of internal object of type BigDecimal

### RSpec
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Finished in 0.00764 seconds
20 examples, 0 failures
```

### Docs
```
Files:           4
Modules:         2 (    0 undocumented)
Classes:         1 (    0 undocumented)
Constants:       1 (    0 undocumented)
Methods:        13 (    0 undocumented)
 100.00% documented
```