fix: proto value unmarshal strings as utf8 string

[muktihari]

Previously, we decode string as it is, even if string in FIT protocol should be a valid utf8 string, there is no guarantee that we got a valid utf8 string when the FIT file is not properly encoded. Now, we will ensure that every character in the string is a valid utf8 character, any invalid character will be replaced with utf8.RuneError ('\uFFFD'/�).

Additional changes:

• Improve UnmarshalValue loop on slice value, this change will reduce bound check.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (ac559cb) to head (618723d).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #247   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           42        42           
  Lines         3615      3628   +13     
=========================================
+ Hits          3615      3628   +13     

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