Optimize _encode_multipart_message (#75)

* Optimize _encode_multipart_message

By using BytesIO while generating the body instead of continuous string
concatenations.

This drastically reduces execution time by not recreating the string for
each concatenation.

In one case, execution time fell from 140 seconds to 0.14; three orders
of magnitude!

* Fix comment about default chunk_size in MB

src/dicomweb_client/web.py

@@ -233,7 +233,7 @@ def __init__(
             Maximum number of bytes that should be transferred per data chunk
             when streaming data from the server using chunked transfer encoding
             (used by ``iter_*()`` methods as well as the ``store_instances()``
-            method); defaults to ``10**6`` bytes (10MB)
+            method); defaults to ``10**6`` bytes (1MB)
 
         Warning
         -------
@@ -743,13 +743,15 @@ def _encode_multipart_message(
             raise ValueError(
                 'No "boundary" parameter in found in content-type field'
             )
-        body = b''
-        for part in content:
-            body += f'\r\n--{boundary}'.encode('utf-8')
-            body += f'\r\nContent-Type: {content_type}\r\n\r\n'.encode('utf-8')
-            body += part
-        body += f'\r\n--{boundary}--'.encode('utf-8')
-        return body
+        with BytesIO() as b:
+            for part in content:
+                b.write(f'\r\n--{boundary}'.encode('utf-8'))
+                b.write(
+                    f'\r\nContent-Type: {content_type}\r\n\r\n'.encode('utf-8')
+                )
+                b.write(part)
+            b.write(f'\r\n--{boundary}--'.encode('utf-8'))
+            return b.getvalue()
 
     @classmethod
     def _assert_media_type_is_valid(cls, media_type: str):