Write tests to improve code coverage

[ErikBjare]

There are a few untested functions in the main Uniswap class, these should be tested: https://app.codecov.io/gh/uniswap-python/uniswap-python/blob/master/uniswap/uniswap.py

Would be a big gain to have all public functions basically covered.

[ErikBjare]

I improved things a bit in #266