Describe the interaction between 100 and 104

Author: guoye-zhang

draft-ietf-httpbis-resumable-upload.md

@@ -681,6 +681,8 @@ The `104 (Upload Resumption Supported)` status code is can be used for two purpo
 - When responding to requests to create uploads, an interim response with the `104 (Upload Resumption Supported)` status code can be sent to indicate the server's support for resumable uploads, as well as the URI and limits of the corresponding upload resource in the `Location` and `Upload-Limit` header fields, respectively (see {{upload-creation}}). This notifies the client early about the ability to resume the upload in case of network interruptions.
 - While processing the content of a request to append representation data or create an upload, the server can regularly send interim responses with the `104 (Upload Resumption Supported)` status code to indicate the current upload progress in the `Upload-Offset` header field (see {{upload-creation}} and {{upload-appending}}). This allows the client to show more accurate progress information about the amount of data received by the server. In addition, clients can use this information to release representation data that was buffered, knowing that it doesn't have to be re-transmitted.
 
+The client MAY treat an interim response with the `104 (Upload Resumption Supported)` status code as fulfilling the 100-continue expectation. However, the server MUST NOT omit the `100 (Continue)` response because it has sent an interim response with the `104 (Upload Resumption Supported)` status code.
+
 # Media Type `application/partial-upload` {#media-type-partial-upload}
 
 The `application/partial-upload` media type describes a contiguous block from the representation data that should be uploaded to a resource. There is no minimum block size and the block might be empty. The block can be a subset of the representation data, where the start and/or end of the block don't line up with the start and/or end of the representation data respectively.