title: Can't insert bytes to big query using pandas-gbq
body: <p>I am using pandas-gbq to insert a simple row into big query.</p>
  <p>My table has only one field define as follow:</p>
  <blockquote>
  <p>data  BYTES   REQUIRED</p>
  </blockquote>
  <p>I am creating my dataframe like this:</p>
  <pre><code>    b64 = base64.b64encode(test.SerializeToString())
      logging.info(b64)
      column = 'data'
      logging.info(type(b64))
      data = {column: [b64]}
      logging.info(data)
      df = pd.DataFrame(data, columns=[column])
  </code></pre>
  <p>and then, I am tryingto insert my data as follow:</p>
  <pre><code>    df.to_gbq(full_table_name, table_schema=[{'name': 'data', 'type': 'BYTES'}],
                        project_id=project, if_exists='append')
  </code></pre>
  <p>I also tried this:</p>
  <pre><code>    pandas_gbq.to_gbq(df, full_table_name, table_schema=[{'name': 'data', 'type': 'BYTES'}],
                        project_id=project, if_exists='append')
  </code></pre>
  <p>Both solution raise this exception:</p>
  <blockquote>
  <p>line 435, in process_http_error raise GenericGBQException(&quot;Reason:
  {0}&quot;.format(ex)) pandas_gbq.gbq.GenericGBQException: Reason: 400 Error
  while reading data, error message: Could not parse
  'b'CsEBCil0eXBlLmdvb2dsZWFwaXMuY29tL2NvbnNvbGUudjEuVGhyZXNob2xkcxKTAQoFFQrXIzw...'
  as BYTES for field data (position 0) starting at location 0 with
  message 'Unable to parse' [while running
  'ParDo(InsertBackUp)-ptransform-2325']</p>
  </blockquote>
  <p>However I am kinf of sure that the data in my dataframe is bytes and I do not understand why the problem comes while reading data as I do not want to read any data.</p>
  <p>I do not know if it's relevant but the table in which I want to insert data is empty.</p>
  <p>Any thoughts ?<br />
  Thanks</p>