Fixes #511: Allow deserialize_string for map fields originating from struct-formatted maps

[grindvoll]

This commit fixes an incoherence problem in the Deserializer implementation for fields in struct-encoded maps. deserialize_str was accepted, while deserialize_string was rejected. This behavior caused a problem when deserializing e.g. serde_json::Values from RON-files.

• I've included my change in CHANGELOG.md

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (1ff0efa) 100.00% compared to head (3dc831f) 100.00%.

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Additional details and impacted files

@@            Coverage Diff            @@
##            master      #512   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           79        80    +1     
  Lines        11109     11178   +69     
=========================================
+ Hits         11109     11178   +69     

Files	Coverage Δ
src/de/id.rs	100.00% <100.00%> (ø)
tests/511_deserialize_any_map_string_key.rs	100.00% <100.00%> (ø)
tests/non_identifier_identifier.rs	100.00% <ø> (ø)

... and 1 file with indirect coverage changes

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[juntyr]

@grindvoll Thanks for the PR! It looks good to me once codecov is 100% and with the small CHANGELOG tweak.