Fix eth.clearSubscriptions size property de-referencing bug

[cgewecke]

Description

The this property is not bound correctly at the web3.eth scope when calling clearSubscriptions:

https://github.com/ethereum/web3.js/blob/740a98258ead66c16b1ec12f3f888baee64694b6/packages/web3-eth/src/index.js#L268

...results in this referring to web3.eth rather than RequestManager when run here:

https://github.com/ethereum/web3.js/blob/740a98258ead66c16b1ec12f3f888baee64694b6/packages/web3-core-requestmanager/src/index.js#L269-L274

• Failing test: commit 1616d2f CI link

Fixes #3527

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.

[coveralls]

Coverage remained the same at 89.622% when pulling 5c60d1f on issue/3257 into 53e4cab on 1.x.

[ryanio]

nice, great. LGTM