Fix param encoding for BN in arrays

[cgewecke]

Description

Moves logic that converts BN/Bignumber to string within formatParam (which recursively unpacks arrayified inputs)

Failing tests commit: 33bbf60

Fixes #3592

Type of change

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

Checklist:

• I have selected the correct base branch.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• Any dependent changes have been merged and published in downstream modules.
• I ran npm run dtslint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:cov and my test cases cover all the lines and branches of the added code.
• I ran npm run build-all and tested the resulting files from dist folder in a browser.
• I have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.

[coveralls]

Coverage remained the same at 90.154% when pulling c114e31 on issue/3592 into db82053 on 1.x.

[ryanio]

LGTM, thanks @cgewecke!