Changelog

Various encoding / decoding fixes:

all field.JSONUnmarshal methods now unquote values using the json.Unmarshal rather than using strconv.Unquote. This is because the json.Unmarshal function is more robust and handles a wider range of edge cases.
field/Binary MarshalJSON and UnmarshalJSON returns hex based strings and takes them in respectively. Previously, they handled bas64 representations of bytes making them pretty unusable in their string formats. This should be regarded to be a breaking change even though the API itself is unchanged.
field/Bitmap no longer supports JSON encoding / decoding. The decision was made to do this because bitmaps add little value in JSON form and are used specifically for the parsing of ISO8583 messages with no business use case.
field/Composite now lazily creates subfields only if they are being packed or unpacked rather than creating all subfields when defined in the spec regardless of whether they are to be populated with values or not. This change was necessary to ensure that only relevant fields are populated in their JSON format. Before, even unpopulated fields were populated with "" or null values. This caused issues when unmarshalling in that JSON representation back into the iso8583.Message.
Replace spec.CreateSubfields with field.CreateSubfield. Due to the change in how Composite now creates fields, we needed to change the function signature of this function.

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