fix for #220 bcd length encoding

[ddvk]

Here's is my proposition for fixing the bcd length encoding. This also fixes the variable encoding where an odd length was encoded with the next even number.

The problem I had with bcd is that there is no way to find out whether the length of original data was odd or even, that's why the Encoder interface, Encode return type change.

I couldn't come up with a more elegant solution.

[adamdecaf]

Thanks for opening this PR! Can you update the godoc comments to mention what that integer represents?

[ddvk]

the godoc of the Encode method?

[adamdecaf]

Yep, thanks.

[adamdecaf]

cc @alovak

Can we fix and amend that typo? @ddvk

[codecov-commenter]

Codecov Report

Patch coverage: 93.47% and project coverage change: +0.01 :tada:

Comparison is base (79404f0) 73.18% compared to head (4c3426a) 73.19%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #221 +/- ## ========================================== + Coverage 73.18% 73.19% +0.01% ========================================== Files 41 41 Lines 1995 1996 +1 ========================================== + Hits 1460 1461 +1 Misses 326 326 Partials 209 209 ``` | [Impacted Files](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io) | Coverage Δ | | |---|---|---| | [encoding/binary.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvYmluYXJ5Lmdv) | `30.00% <0.00%> (ø)` | | | [encoding/ebcdic1047.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvZWJjZGljMTA0Ny5nbw==) | `76.47% <66.66%> (ø)` | | | [encoding/ascii.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvYXNjaWkuZ28=) | `89.47% <100.00%> (ø)` | | | [encoding/bcd.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvYmNkLmdv) | `100.00% <100.00%> (ø)` | | | [encoding/bertlv.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvYmVydGx2Lmdv) | `90.90% <100.00%> (-0.40%)` | :arrow_down: | | [encoding/ebcdic.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvZWJjZGljLmdv) | `100.00% <100.00%> (ø)` | | | [encoding/hex.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvaGV4Lmdv) | `87.09% <100.00%> (ø)` | | | [encoding/lbcd.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZW5jb2RpbmcvbGJjZC5nbw==) | `100.00% <100.00%> (ø)` | | | [field/binary.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZmllbGQvYmluYXJ5Lmdv) | `60.67% <100.00%> (ø)` | | | [field/bitmap.go](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io#diff-ZmllbGQvYml0bWFwLmdv) | `74.19% <100.00%> (ø)` | | | ... and [11 more](https://codecov.io/gh/moov-io/iso8583/pull/221?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=moov-io) | | 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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[alovak]

@ddvk thanks for raising the issue and your contributions. You helped us to realize that there is an issue with field packing/unpacking. I'm closing this PR as the issue is going to be addressed by this PR: https://github.com/moov-io/iso8583/pull/227

[ddvk]

sure, thanks a lot.