Allow WithPath to work for floating point numbers

[alvii147]

#449 details the problem. The fix was to handle the cases of float32 and float64 separately, and use strconv.FormatFloat instead of fmt.Sprint in those cases.

I've also added a couple of test cases to verify it works and doesn't break this for other types.

[coveralls]

coverage: 94.815% (+0.002%) from 94.813%
when pulling db112b3 on alvii147:alvii147/449/WithPath-floating-point
into 07a4dbe on gavv:master.