toUrlPiece and parseUrlPiece are not reversible

```haskell
x :: Double
x = 9.0e3

t :: Text
t = toUrlPiece x
-- t == "9.0e-3"

x' :: Either Text Double
x' = parseUrlPiece t
-- x' == Right 9.000000000000001e-3

isReversible :: Bool
isReversible = Right x == x'
-- isReversible == False
```

The underlying issue is probably in `rational :: Fractional a => Reader a` not being compatible with `show :: Double -> String`.