add missing int64 support for String field Unmarshal

[alovak]

We forgot to handle the int64 type when we Unmarshal field. The following code returned the error unsupported reflect.Value type: int64:

	data := struct {
		MTI               string `index:"0"`
		TransactionAmount int64  `index:"4"`
	}
	err := msg.Unrashal(&data)
	// handle error

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6ecba23) 74.86% compared to head (6fe69f3) 74.98%.

❗ Current head 6fe69f3 differs from pull request most recent head 54b929f. Consider uploading reports for the commit 54b929f to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #303      +/-   ##
==========================================
+ Coverage   74.86%   74.98%   +0.12%     
==========================================
  Files          44       44              
  Lines        2495     2495              
==========================================
+ Hits         1868     1871       +3     
+ Misses        411      407       -4     
- Partials      216      217       +1     

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