Fix toWei(0.1,'wei') from returning 1

[junderw]

Fixes #1 and also adds tests for the 2 missing branches (unrelated to this PR) so now coverage is 100% across the board.

[junderw]

oof... looks like the dependencies I pulled in are newer than master, so a large diff was created in the pre-commit build...

I'll try to find the version being used here.

[junderw]

ok, nvm. all dependencies are specific versions...

Not quite sure how to deal with the built source... Should I maybe just remove them?

This was installed + built using the following node versions btw:

$ node -v
v12.18.3
$ npm -v
6.14.8

[coveralls]

Coverage remained the same at 100.0% when pulling fc89e67 on junderw:fixToWei into 35d870e on ethjs:master.

[SilentCicero]

Reviewing.