Make user supplied callback fire in accounts.signTransaction

[cgewecke]

Description

Fixes a bug which disabled the ability to call eth.accounts.signTransaction using the callback form.

@michaelsbradleyjr This only addresses the narrowest part of your issue. Have not used Object.assign as you suggest there, mostly to keep with the pattern of Web3's existing code. Please advise if you believe that's critical though.

Fixes #3283

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 have updated the CHANGELOG.md file in the root folder.
• I have tested my code on the live network.

[coveralls]

Coverage increased (+0.07%) to 84.875% when pulling e9c715c on issue/3283 into 58f77a6 on 1.x.

[michaelsbradleyjr]

Looks good, and sorry if I didn't communicate clearly when I mentioned "assign":

...it seems like it should assign the object to result

By that I meant exactly what you did (though see my comment below re: declaration) and not Object.assign.

[nivida]

LGTM!