Fix marshalling of CustomGenesis

[otherview]

Description

Fix for genesis.CustomGenesis serialization. The (i *HexOrDecimal256) MarshalJSON() and the (b *Bytes32) MarshalJSON() were working incorrectly. This was found when the networkHub was trying to write genesis.CustomGenesis to file and thor nodes were loading the genesis back. Both operations were erroring with different reasons.

• HexOrDecimal256 MarshalJSON was returning a String instead of a Json object

• JSONMarshalling of HexOrDecimal256 would fail because a string is not a JSON ( needs to be "string" )

• Bytes32 MarshalJSON was returning a JSON string of an array of an hex encoded string instead of a JSON string of the hex encoded string

• JSONUnmarshalling of Bytes32 would fail because it would expect a JSON string of Hex enconded value ( expected "0x00000..." received from the Marshaller "[0 12 0 ... 0]"

• As an interesting note: Nilled or zeroedBytes32 variable ( zero := Bytes32{} ) always marshalls to 0x000... instead of nil. This PR does not change this as it might create upstream issues.

The PR also:

• Fixes the unit tests for HexOrDecimal256
• Adds test for Bytes32 Marshal/Unmarshal and genesis.CustomGenesis Marshal

Type of change

Please delete options that are not relevant.

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Added tests

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing E2E tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 61.37%. Comparing base (88baecd) to head (0849209). Report is 1 commits behind head on master.

Files	Patch %	Lines
genesis/customnet.go	66.66%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #730 +/- ## ========================================== + Coverage 61.34% 61.37% +0.03% ========================================== Files 194 194 Lines 18221 18223 +2 ========================================== + Hits 11177 11184 +7 + Misses 5961 5958 -3 + Partials 1083 1081 -2 ```

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[otherview]

Was able to confirm that this does not generate any errors in a running network.

Ran the following test:

• Spawn a network with 6 nodes ( 2 Auth + 1 Public nodes in the latest version and 2 Auth + 1 Public nodes with this branch )
• Ran thor-e2e-test against both nodes running latest and nodes running this branch.
• Verified same results and no outstanding errors.

I think it looks good to merge.