fix(api): ensure memtable schema and columns match

[deepyaman]

Description of changes

Before this change, if you pass a SchemaLike that isn't actually a Schema, it won't automatically rename the auto-generated pandas column names. For example:

import pandas as pd

df = pd.DataFrame([[1, 2], [3, 4]])
schema = {"a": "int64", "b": "int64"}
t = ibis.memtable(df, schema=schema)
t.a.execute()  # Raises a KeyError

This PR goes ahead and constructs the Schema, if passed—work that was going to be done anyway. That way, getattr(schema, "names") finds the passed names, and the rename is performed.

[cpcloud]

You need to monkey patch the default backend setting

[deepyaman]

You need to monkey patch the default backend setting

Where?

(Also, why? Did I cause the nix failure; doesnt seem related to my change, even though its related to the default backend?)

[cpcloud]

You need to monkey patch the default backend setting

Where?

(Also, why? Did I cause the nix failure; doesnt seem related to my change, even though its related to the default backend?)

The default backend attribute is global state, and tests run in a random order. This means that any of our suites could have failed in the way, you just got lucky 😄