Fix parsing of includefield query parameters

There can be more than one ``includefield`` key-value pair in the query
string.

src/dicomweb_client/api.py

@@ -256,10 +256,11 @@ def _parse_query_parameters(self, fuzzymatching, limit, offset,
             else:
                 params['fuzzymatching'] = 'false'
         if fields is not None:
+            params['includefield'] = list()
             for field in set(fields):
                 if not(isinstance(field, str)):
                     raise TypeError('Elements of "fields" must be a string.')
-                params['includefield'] = field
+                params['includefield'].append(field)
         for field, criterion in search_filters.items():
             if not(isinstance(field, str)):
                 raise TypeError('Keys of "search_filters" must be strings.')

src/dicomweb_client/tests/test_api.py

@@ -99,6 +99,23 @@ def test_search_instances_limit_offset(httpserver, client, cache_dir):
     assert request.accept_mimetypes == [('application/dicom+json', 1)]
 
 
+def test_search_instances_includefields(httpserver, client, cache_dir):
+    headers = {'content-type': 'application/dicom+json'}
+    httpserver.serve_content(content='', code=200, headers=headers)
+    f1 = 'StudyInstanceUID'
+    f2 = 'SeriesInstanceUID'
+    client.search_instances(fields={f1, f2})
+    request = httpserver.requests[0]
+    query_string_opt_1 = 'includefield={}&includefield={}'.format(f1, f2)
+    query_string_opt_2 = 'includefield={}&includefield={}'.format(f2, f1)
+    assert (
+        request.query_string.decode() == query_string_opt_1 or
+        request.query_string.decode() == query_string_opt_2
+    )
+    assert request.path == '/instances'
+    assert request.accept_mimetypes == [('application/dicom+json', 1)]
+
+
 def test_retrieve_instance_metadata(httpserver, client, cache_dir):
     cache_filename = os.path.join(cache_dir, 'retrieve_instance_metadata.json')
     with open(cache_filename, 'r') as f: