fixed accounts.sign invalid length because of missing leading zero

[emn178]

Description

Sometimes r and s should have leading zero, but current version has bug and return invalid length, for example:

{
  message: '0xc1af4b94166cd32fc49b7b926cbb91ee421de2d04450e8ae57857b9b56ac7e53',
  messageHash: '0x69a1658d9d5e169959e0e2acec0eb76ec15549ebc0680f45cfc25c4a3ca952e6',
  v: '0x1c',
  r: '0x3105c6f9bcc43236f5ef5e78038506c2551fc11d65701097295094c06c9d4f3',
  s: '0x29ec14347d37d8115cebcf915dc8f6f74c011698d9b7aea15c184f2277197a',
  signature: '0x03105c6f9bcc43236f5ef5e78038506c2551fc11d65701097295094c06c9d4f30029ec14347d37d8115cebcf915dc8f6f74c011698d9b7aea15c184f2277197a1c'
}

It should be

{
  message: '0xc1af4b94166cd32fc49b7b926cbb91ee421de2d04450e8ae57857b9b56ac7e53',
  messageHash: '0x69a1658d9d5e169959e0e2acec0eb76ec15549ebc0680f45cfc25c4a3ca952e6',
  v: '0x1c',
  r: '0x03105c6f9bcc43236f5ef5e78038506c2551fc11d65701097295094c06c9d4f3',
  s: '0x0029ec14347d37d8115cebcf915dc8f6f74c011698d9b7aea15c184f2277197a',
  signature: '0x03105c6f9bcc43236f5ef5e78038506c2551fc11d65701097295094c06c9d4f30029ec14347d37d8115cebcf915dc8f6f74c011698d9b7aea15c184f2277197a1c'
}

In old version, e.g. 1.x it returns correct result like above.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

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 lint with success and extended the tests and types if necessary.
• I ran npm run test:unit with success.
• I ran npm run test:coverage and my test cases cover all the lines and branches of the added code.
• I ran npm run build and tested dist/web3.min.js in a browser.
• I have tested my code on the live network.
• I have checked the Deploy Preview and it looks correct.
• I have updated the CHANGELOG.md file in the root folder.
• I have linked Issue(s) with this PR in "Linked Issues" menu.

[luu-alex]

Thanks for the PR! can you update the changelog as well

[codecov]

Codecov Report

Merging #6411 (9879a8e) into 4.x (3a9f7d7) will not change coverage.
Report is 1 commits behind head on 4.x.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##              4.x    #6411   +/-   ##
=======================================
  Coverage   89.11%   89.11%           
=======================================
  Files         199      199           
  Lines        7723     7723           
  Branches     2128     2128           
=======================================
  Hits         6882     6882           
  Misses        841      841           

Flag	Coverage Δ
UnitTests	89.11% <100.00%> (ø)

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Components	Coverage Δ
web3	∅ <ø> (∅)
web3-core	∅ <ø> (∅)
web3-errors	∅ <ø> (∅)
web3-eth	∅ <ø> (∅)
web3-eth-abi	∅ <ø> (∅)
web3-eth-accounts	∅ <ø> (∅)
web3-eth-contract	∅ <ø> (∅)
web3-eth-ens	∅ <ø> (∅)
web3-eth-iban	∅ <ø> (∅)
web3-eth-personal	∅ <ø> (∅)
web3-net	∅ <ø> (∅)
web3-providers-http	∅ <ø> (∅)
web3-providers-ipc	∅ <ø> (∅)
web3-providers-ws	∅ <ø> (∅)
web3-rpc-methods	∅ <ø> (∅)
web3-utils	∅ <ø> (∅)
web3-validator	∅ <ø> (∅)