Fix WSS credentials Base64 encoding (for browser)

[cgewecke]

Description

Binds btoa to window correctly.

const _web3 = new Web3('wss://usr:psswrd@localhost:8546)

Currently the example above throws these errors in the browser:

Chrome

TypeError: Illegal invocation
    at new s (/var/folders/_p/1jdl7zfx1473cwzhqsz8gx4r0000gn/T/880f08d03d5a7b63dd93c6a4c8d3d895.browserify.js:125206:1235495)

Firefox

'btoa' called on an object that does not implement interface Window.
	s@/var/folders/_p/1jdl7zfx1473cwzhqsz8gx4r0000gn/T/3c102c58b323c6dbbf38414c1423b68e.browserify.js:125206:1235164

• 17e9671 adds a test that can be seen failing here
• 9be703d is the fix proposed by @vsleibenko in Websocket Provider's btoa helper's context [browser only] #3508

Fixes #3508

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 89.555% when pulling 488b7c2 on issue/3508 into 697b40f on 1.x.

[ryanio]

LGTM! Thanks for the test case.