title: Seeing error: session must be <requests.session.Session object> not <class 'str'> - Not sure what it means
body: <p>I am experimenting with IG Markets and trying to run the below: </p>
  
  <pre><code>response = ig_service.fetch_client_sentiment_by_instrument('GBPUSD','NZDGBP')
  print(response) 
  </code></pre>
  
  <p>But I see this error: </p>
  
  <p>File "C:\Users\Barry\Desktop\IG Trading\ig-markets-api-python-library-master\trading_ig\rest.py", line 211, in _get_session
      % type(session)
  AssertionError: session must be  not </p>
  
  <p>Please note, it only happens when there is more than one marketid used: ('GBPUSD','NZDGBP').The request below works: </p>
  
  <pre><code>response = ig_service.fetch_client_sentiment_by_instrument('GBP')
  print(response)   
  </code></pre>
  
  <p>Code below shows actual declarations</p>
  
  <pre><code>def fetch_client_sentiment_by_instrument(self, market_id, session=None):
      """Returns the client sentiment for the given instrument's market"""
      params = {}
      if isinstance(market_id, (list,)):
          market_ids = ','.join(market_id)
          url_params = {
              'market_ids': market_ids
          }
          endpoint = '/clientsentiment/?marketIds={market_ids}'.\
              format(**url_params)
      else:
          url_params = {
              'market_id': market_id
          }
          endpoint = '/clientsentiment/{market_id}'.format(**url_params)
      action = 'read'
      response = self._req(action, endpoint, params, session)
      data = self.parse_response(response.text)
      if self.return_munch:
          from .utils import munchify
          data = munchify(data)
      return(data)
  
  
  def _get_session(self, session):
      """Returns a Requests session (from self.session) if session is None
      or session if it's not None (cached session with requests-cache
      for example)
      """
      if session is None:
          session = self.session  # requests Session
      else:
          assert(isinstance(session, Session)), \
              "session must be &lt;requests.session.Session object&gt; not %s" \
              % type(session)
          session = session
      return session
  </code></pre>