Axios added duplicated content-length header when request already has a content-length header

[XiaoningLiu]

Describe the bug
Axios is a promise based HTTP client with browser and Node.js support. As an HTTP client, Axios wants to make sure the sent message contains a proper content-length header as well. So it includes a number type “Content-Length” header by default. This behavior can be observed in /node_modules/axios/lib/adapters/http.js, at line 45.

If request already has a lowercase "content-length" header, since they have different casing, the header included by Axios does not overwrite the one included by the request. So we’re sending two “content-length” headers to the server (same values and different types). I believe this is not the expected behavior we want.

Expected behavior

Before set content-length header, Axios should check if there is a case insensitive "content-length" header.

IF THERE IS AN EXISTING CONTENT-LENGTH HEADER:
COMPARE WITH CONTENT-LENGTH CALCULATED
IF DISMATCH
THROW ERROR
ELSE
SET CONTENT-LENGTH HEADER IN AXIOS

Environment:

• Axios Version 0.18.0
• OS: Any
• Node.js: Any