toWei: incorrect result for small numbers (< 1e-6)

[slavafomin]

Issue Type

• Bug (https://github.com/ethjs/ethjs-unit/blob/master/.github/CONTRIBUTING.md#bug-reports)
• Feature (https://github.com/ethjs/ethjs-unit/blob/master/.github/CONTRIBUTING.md#feature-requests)

Description

The toWei() function will return incorrect results when passing numbers smaller than 1e-6 to it.

For example toWei(1e-7, 'gwei').toString() will return 2377000000000 instead of 100.

The issue starts here and then cascades downward. The problem is that you are using the String constructor to convert number to a string. However, it will produce some small numbers in exponential notation instead of the fixed notation that you are expecting.

Steps to reproduce

Check the PR #7.

Versions

Doesn't matter.

[slavafomin]

Check the PR #7 for a fix.