Task 339 make to_f on eny passed value

[Task 339] (https://integracredit.acunote.com/tasks/339) - rewrite Monetize to use decimal math as a new gem

### Changes
* Make to_f on passed value for contructor

### RSpec
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Finished in 0.00485 seconds
8 examples, 0 failures
```

### Docs
```
Files:           3
Modules:         1 (    0 undocumented)
Classes:         1 (    0 undocumented)
Constants:       1 (    0 undocumented)
Methods:         5 (    0 undocumented)
 100.00% documented
```