fixed bug with is_transformer

[seanmacavaney]

Related to #258 -- subclasses of pt.Transformer doesn't pass is_transformer check because it checks for TransformerBase (a subclass of Transformer).

This means that you wouldn't be able to use a subclass of pt.Transformer in a pipeline. E.g.,

class MyModel(pt.Transformer):
    pass
bm25 >> MyModel() # would fail with message saying MyModel cannot be coerced into a transformer

Since most code still uses TransformerBase, it doesn't break existing code -- just new stuff written using the new name.

Added tests that failed with previous impl and added fix.

[seanmacavaney]

One of the checks failed, but it just seems to be a network error.

[cmacdonald]

yes. enough passed that I was comfortable