exchange serialization bugfix and partial test coverage

[julian-CStack]

No description provided.

[codecov-commenter]

Codecov Report

Merging #10 (6b534a4) into staging (7043ccc) will increase coverage by 1.63%.
The diff coverage is 46.15%.

@@ Coverage Diff @@## staging #10 +/- ##
===========================================
+ Coverage 40.91% 42.54% +1.63% 
===========================================
Files 100 100 Lines 10840 10842 +2 ===========================================
+ Hits 4435 4613 +178 + Misses 6405 6229 -176 

Impacted Files	Coverage Δ
lib/services/change_now/change_now.dart	59.00% <46.15%> (+59.00%)	⬆️
...odels/exchange/change_now/change_now_response.dart	42.85% <0.00%> (+28.57%)	⬆️
...dels/exchange/change_now/exchange_transaction.dart	31.57% <0.00%> (+31.57%)	⬆️
lib/models/exchange/change_now/currency.dart	32.43% <0.00%> (+32.43%)	⬆️
.../models/exchange/change_now/fixed_rate_market.dart	33.33% <0.00%> (+33.33%)	⬆️
...exchange/change_now/estimated_exchange_amount.dart	73.07% <0.00%> (+73.07%)	⬆️

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