BUG: Fix Bokeh tooltip showing literal ' '

Noticed with Bokeh 2.2.1

Author: kernc

backtesting/_plotting.py

@@ -234,7 +234,7 @@ def plot(*, results: pd.Series,
 return this.labels[index] || "";
         ''')
 
-    NBSP = ' ' * 4
+    NBSP = '\N{NBSP}' * 4
     ohlc_extreme_values = df[['High', 'Low']].copy(deep=False)
     ohlc_tooltips = [
         ('x, y', NBSP.join(('$index',