title: pandas merge_asof: ambiguous argument types error
body: <p>I'm trying to use <code>merge_asof</code> from Pandas and I've been getting the error:</p>
  
  <p><code>TypeError: Function call with ambiguous argument types</code></p>
  
  <p>Reproducible example:</p>
  
  <pre><code>import pandas as pd
  
  a = pd.DataFrame({'foo': [1., 2.], 'bar': ['2019-01-01 00:00:10', '2019-01-01 00:00:20']})
  b = pd.DataFrame({'foo': [2., 5.], 'baz': ['2019-01-01 00:00:05', '2019-01-01 00:00:25']})
  a['bar'] = pd.to_datetime(a['bar'])
  b['baz'] = pd.to_datetime(b['baz'])
  
  pd.merge_asof(a,
                b,
                left_on='bar',
                right_on='baz',
                direction='backward',
                by='foo',
                allow_exact_matches=False)
  </code></pre>
  
  <p>I've tried to inspect the pandas.core.reshape.merge file but had no luck solving the problem</p>