add coverage for bm25 fit

tests/unit/encoders/test_bm25.py

@@ -10,7 +10,14 @@ def bm25_encoder():
 
 class TestBM25Encoder:
     def test_initialization(self, bm25_encoder):
-        bm25_encoder = BM25Encoder()
+        assert len(bm25_encoder.idx_mapping) != 0
+
+    def test_fit(self, bm25_encoder):
+        bm25_encoder.fit([
+            "some docs",
+            "and more docs",
+            "and even more docs"
+        ])
         assert len(bm25_encoder.idx_mapping) != 0
 
     def test_call_method(self, bm25_encoder):