title: AttributeError: 'DataFrame' object has no attribute '_example'
body: <p>I am trying to join a few geodataframes using the Dask python package. 
  While implementing my data processing algorithm I faced up with the next exception:
      AttributeError: 'DataFrame' object has no attribute '_example'</p>
  
  <p>Here is my code: </p>
  
  <pre><code>import dask.dataframe as dd
  import dask_geopandas as dg
  import pandas as pd
  import dask
  
  
  df1= gpd.read_file("shapefile1.shp")
  df2= gpd.read_file("shapefile2.shp")
  
  df1= dd.from_pandas(df1, npartitions=1)
  df2= dd.from_pandas(df2, npartitions=1)
  
  gf2 = dg.sjoin(df1, df2, how='inner', op='intersects')
  </code></pre>
  
  <p>Here is my stacktrace:</p>
  
  <pre><code>Traceback (most recent call last):
    File "test.py", line 21, in &lt;module&gt;
      gf2 = dg.sjoin(df1, df2, how='inner', op='intersects')
    File "/usr/local/lib/python3.6/dist-packages/dask_geopandas-0.0.1-py3.6.egg/dask_geopandas/core.py", line 413, in sjoin
      example = gpd.tools.sjoin(left._example, right._example, how=how, op=op)
    File "/home/mapseeuser/.local/lib/python3.6/site-packages/dask/dataframe/core.py", line 2414, in __getattr__
      raise AttributeError("'DataFrame' object has no attribute %r" % key)
  AttributeError: 'DataFrame' object has no attribute '_example'
  </code></pre>
  
  <p>So, could anybody tell me what I am doing wrong and how to join two datasets using Dask package library.  </p>