Update Transaction to avoid serde errors

[BreD1810]

This PR:

• Removes account_balance field on Transaction - although this is still shown on the Monzo doc examples, it does not appear to be returned by the API any more?
• Changes Category from an enum to a String (as suggested in transaction categories are not exhaustive #12).
• Makes the ID variant of MerchantInfo an Option<String> (the API can return null values, for example with a top-up).

[codecov]

Codecov Report

Merging #16 (6f4edf7) into master (1f412da) will increase coverage by 3.72%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##           master     #16      +/-   ##
=========================================
+ Coverage    3.57%   7.29%   +3.72%     
=========================================
  Files          17      18       +1     
  Lines         224     233       +9     
=========================================
+ Hits            8      17       +9     
  Misses        216     216              

Impacted Files	Coverage Δ
src/endpoints/transactions.rs	100.00% <100.00%> (ø)

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[danieleades]

is it possible to add some unit tests to show the deserialisation works? I would recommend grabbing the raw JSON from the API playground, anonymising it, and then showing that it's correctly deserialised.

you would have to extract the logic in the send method into a function that you can unit test.

[BreD1810]

is it possible to add some unit tests to show the deserialisation works?

I've added (anonymised) unit tests for:

• Expanded transaction
• Transaction with null merchant
• Multiple transactions in a single request.

[danieleades]

i'm happy with these changes. so long as you promise the test data is properly anonymised. I'd hate to inadvertently share any personal information

[BreD1810]

Triple checked none of the identifying values are actual ones - just tried to keep them similar to actual values that may be returned.

I essentially used ascending numbers/letters for each value, making sure the digits were in the correct location as returned by the real response. Eg:

"id": "tx_0000A1aBC2Dbc34Ede5fEH",

[danieleades]

brilliant work @BreD1810. Thanks again for the pull request!