Fix binaryVarPrefixer to work with more bytes than prefix

[FacundoMora]

Adjusts the length decoding, the previous implementation only works when the data provided only has the length bytes, being the real case scenario when data always haves more data than the prefix bytes.

To adjust this issue was necessary to cut the amount of bytes corresponding to the digits attribute from the data provided.

[adamdecaf]

Thanks for fixing this @FacundoMora!

[codecov-commenter]

Codecov Report

Base: 73.29% // Head: 73.31% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (80b18fa) compared to base (bb0b3da). Patch coverage: 100.00% of modified lines in pull request are covered.

:exclamation: Current head 80b18fa differs from pull request most recent head 358f6c0. Consider uploading reports for the commit 358f6c0 to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #212 +/- ## ========================================== + Coverage 73.29% 73.31% +0.01% ========================================== Files 41 41 Lines 1985 1986 +1 ========================================== + Hits 1455 1456 +1 Misses 325 325 Partials 205 205 ``` | [Impacted Files](https://codecov.io/gh/moov-io/iso8583/pull/212?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io) | Coverage Δ | | |---|---|---| | [prefix/binary.go](https://codecov.io/gh/moov-io/iso8583/pull/212?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-cHJlZml4L2JpbmFyeS5nbw==) | `64.00% <100.00%> (+0.73%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io)

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