server: strip content-length header on proxy

[ngxson]

fix #17710

Note: there is also another alternative fix is to allow server_http_proxy to select if it wants to return a stream
or not. but IMO this solution is not elegant because in case of non-stream:

• it is difficult to track timeout errors
• proxy with large amount of data becomes slow
• higher memory usage
• add lot more code

[pwilkin]

Just an idea - wouldn't it be a better just to sanitize headers at the end and then just remove the duplicates? Or does the proxy add duplicates on its own?

[ngxson]

Just an idea - wouldn't it be a better just to sanitize headers at the end and then just remove the duplicates? Or does the proxy add duplicates on its own?

some headers are added by httplib, so we don't actually have direct control over what headers are added.

btw, the headers type is already std::set, so duplicate is not even allowed.

[ngxson]

just to note that, adding content-length doesn't result in duplicated header, but some HTTP clients are quite strict and doesn't allow content-length to be specified in stream mode (chunk encoding), so we have to remove it here

for example, most browsers don't care about this, but nginx throws an error