Change conn.conn_kw's content instead of replacing it (Azure#16587)

* conn.conn_kw

* Update _bigger_block_size_http_adapters.py

Co-authored-by: Xiang Yan <xiangsjtu@gmail.com>

sdk/core/azure-core/azure/core/pipeline/transport/_bigger_block_size_http_adapters.py

@@ -41,5 +41,7 @@ def get_connection(self, url, proxies=None):
         conn = super(BiggerBlockSizeHTTPAdapter, self).get_connection(url, proxies)
         system_version = tuple(sys.version_info)[:3]
         if system_version[:2] >= (3, 7):
-            conn.conn_kw = {"blocksize": 32768}
+            if not conn.conn_kw:
+                conn.conn_kw = {}
+            conn.conn_kw['blocksize'] = 32768
         return conn