fix: ignore non-canonical asset names when scaling

[ascandone]

Reported by @Azorlogh on #199.

funds.GetAssets groups an account's balances by scale, but different asset names can have the same scale: EUR and EUR/0 are both scale 0. One balance overwrote the other, and which one was kept depended on map iteration order.

With balances acc1: EUR = 5, EUR/0 = 0, EUR/2 = 100, this script:

send [EUR 1] (
  source = @acc1 with scaling through @swap
  destination = @dest
)

sometimes pays the 1 EUR directly and sometimes converts EUR/2 through @swap, because the EUR/0 balance hides the 5 EUR.

Scaling postings always use the canonical name (EUR, EUR/2, never EUR/0). A balance whose name isn't in that form can't be spent under the name the posting uses. GetAssets now skips those balances (EUR/0, EUR/02, EUR/x...). Adding them to the canonical balance instead would spend EUR the account doesn't have.

Tests:

• a unit test for GetAssets with non-canonical names
• a new case in scaling.num.specs.json: EUR = 4, EUR/0 = 0 must convert the 4 EUR (it failed every time before the fix)

[NumaryBot]

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