title: drf_yasg doesn't take TYPE_ARRAY as a valid type
body: <p>the drf_yasg swagger generator doesnt take TYPE_ARRAY as a valid parameter type.
  the implementation is as follows</p>
  
  <pre><code>from drf_yasg import openapi
  param1 = openapi.Parameter('param_name',
                              in_=openapi.IN_QUERY,
                              description='description of param',
                              type=openapi.TYPE_ARRAY,
                              required=True
                              )
  </code></pre>
  
  <p>whereas the <a href="https://media.readthedocs.org/pdf/drf-yasg/latest/drf-yasg.pdf" rel="nofollow noreferrer">documentation</a> of drf_yasg suggests that it takes openapi.TYPE_ARRAY as valid type.</p>
  
  <p>the error that the generators throws is </p>
  
  <pre><code>File "/usr/local/lib/python3.6/dist-packages/drf_yasg/codecs.py", line 73, in encode
      raise SwaggerValidationError("spec validation failed", errors, spec, self)
  drf_yasg.errors.SwaggerValidationError: spec validation failed
  </code></pre>
  
  <p>is there some configuration that i am missing or something because TYPE_STRING,TYPE_NUMBER works perfectly fine.</p>