unexpected behavior in cached function when key words arguments are used.

Hello,

I have found this project very useful and while I was doing tests I have found this behavior unexpected for me.

I leave below a minimum reproducible example:

```
import pandas as pd
import cachey

df = pd.DataFrame([[1, 2]], columns=list("ab"))
c = cachey.Cache(1e9)

@c.memoize
def boo(df):
print("not cached!")
return df, df.to_dict()

boo(df=df) # not works
boo(df) # works
boo(df) # works

```

I don't know if this comment is appropriate but I thought it was interesting, maybe you can give me a clue as to what could cause this problem.

Start by running the minimal example from the issue, then inspect Cache.memoize and the cache-key handling it uses for positional and keyword arguments. Done means equivalent calls such as boo(df=df) and boo(df) consistently share the cached result, with the behavior covered by a regression test.

Written by the indexing model from the issue text.