add repsertBy functionality

With the advent of AtLeastOneUniqueKey and newer methods that rely on it, there is one function missing from the suite: repsertBy.

I propose adding this method and making it analogous to the already existing insertBy:

repsertBy 
    ::
    ( MonadIO m
    , PersistUniqueWrite backend
    , PersistRecordBackend record backend 
    , AtLeastOneUniqueKey record
    )
    => record -> ReaderT backend m ()
repsertBy val = do 
    res <- getByValue val 
    case res of 
        Nothing -> insert_ val 
        Just (Entity key _) -> replace key val 

Happy to submit a PR to this effect!

Start with the existing insertBy entry point in Database.Persist.Class and review the AtLeastOneUniqueKey-based methods. Check how getByValue, insert_, and replace are used, then add matching coverage in the existing test suite. Done means repsertBy handles both absent and present unique-key results with the proposed behavior.