check for the max length in BerTLV prefix

In the spec I'm working with now, there is a max length for BerTLV fields.

Currently, we are ignoring the Length (max length) of the field spec for prefix.BerTLV. This PR changes the prefix a little bit:

if Length is set to 0 or is not set (0 is zero value used for the Length anyway) - we do not check max length - this is to support current integrations which, most probably, follow the example from our tests like this:

            Subfields: map[string]field.Field{
                "9A": field.NewString(&field.Spec{
                    Description: "Transaction Date",
                    Enc:         encoding.Binary,
                    Pref:        prefix.BerTLV,
                }),
                "9F02": field.NewString(&field.Spec{
                    Description: "Amount, Authorized (Numeric)",
                    Enc:         encoding.Binary,
                    Pref:        prefix.BerTLV,
                }),
            },

if Length is set to non-zero value, perform the check

This PR is a breaking change (potentially) if you use the prefix.BerTLV and set non-zero Length in the field spec.

Codecov Report

Patch coverage: 75.00% and project coverage change: +0.05% :tada:

Comparison is base (026d31c) 73.43% compared to head (93bf5ae) 73.49%. Report is 1 commits behind head on master.

:exclamation: Current head 93bf5ae differs from pull request most recent head 105736b. Consider uploading reports for the commit 105736b to get more accurate results

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #256 +/- ## ========================================== + Coverage 73.43% 73.49% +0.05% ========================================== Files 43 43 Lines 2270 2290 +20 ========================================== + Hits 1667 1683 +16 - Misses 373 375 +2 - Partials 230 232 +2 ``` | [Files Changed](https://app.codecov.io/gh/moov-io/iso8583/pull/256?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io) | Coverage Δ | | |---|---|---| | [field/composite.go](https://app.codecov.io/gh/moov-io/iso8583/pull/256?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZmllbGQvY29tcG9zaXRlLmdv) | `83.22% <60.00%> (ø)` | | | [field/bitmap.go](https://app.codecov.io/gh/moov-io/iso8583/pull/256?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZmllbGQvYml0bWFwLmdv) | `71.05% <75.00%> (+0.29%)` | :arrow_up: | | [prefix/bertlv.go](https://app.codecov.io/gh/moov-io/iso8583/pull/256?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-cHJlZml4L2JlcnRsdi5nbw==) | `87.87% <83.33%> (-4.13%)` | :arrow_down: | | [message.go](https://app.codecov.io/gh/moov-io/iso8583/pull/256?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-bWVzc2FnZS5nbw==) | `71.30% <100.00%> (+0.52%)` | :arrow_up: |

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moov-io / iso8583

check for the max length in BerTLV prefix #256

Codecov Report