Fix trustline updates with transfer of "0"

[compojoom]

If we are triyng to update a trustline and we pass a transfer value of "0"(string) the clientlib will call the overloaded updateTrustline function of the contract. This causes the meta transaction to fail as there is already an existing trustline and when updating we shouldn't be passing a transfer value.

[cducrest]

I think this is wrong, you don't allow for strings that are over the number limit for javascript. I would say we need to use something like calcRaw and then compare with bigNumber(0) https://github.com/trustlines-protocol/clientlib/blob/8b30ce108a7e5ec6e90ff3359c2c248ee8a330d4/src/utils.ts#L170

[compojoom]

yes, yes. You are right. That was what I thought on my way to work this morning... I'll have to figure something else.

[codecov[bot]]

Codecov Report

Merging #424 (d867cf1) into master (8b30ce1) will decrease coverage by 0.11%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
- Coverage   59.14%   59.03%   -0.12%     
==========================================
  Files          21       21              
  Lines        1503     1506       +3     
  Branches      314      316       +2     
==========================================
  Hits          889      889              
- Misses        539      540       +1     
- Partials       75       77       +2     

Flag	Coverage Δ
integration	32.07% <0.00%> (-0.07%)	:arrow_down:
unit	59.03% <0.00%> (-0.12%)	:arrow_down:

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Impacted Files	Coverage Δ
src/Trustline.ts	67.78% <0.00%> (-1.40%)	:arrow_down:

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