`httpx` does not allow header values with trailing spaces

[perkfly]

httpx.get("http://example.com", headers={"User-Agent": "Mozilla/5.0 (Windows NT 6.1; WOW64) Gecko/20100101 Firefox/63.0 "})  # note the trailing space
LocalProtocolError: Illegal header value b'Mozilla/5.0 (Windows NT 6.1; WOW64) Gecko/20100101 Firefox/63.0 '

requests.get("http://example.com", headers={"User-Agent": "Mozilla/5.0 (Windows NT 6.1; WOW64) Gecko/20100101 Firefox/63.0 "})
<Response [200]>

[uriyyo]

Hi @yeraydiazdiaz,

Actually, it is not a problem with httpx.
h11 library raise this exception so I think this issue should be transferred from httpx to h11.

[lovelydinosaur]

So, h11 is raising an error here because trailing spaces in HTTP headers is a spec violation.

(I mean, I haven't double checked that yet, but it's almost certainly why)

The first question to ask would be - do you actually need this, and if so why?

Presumably the sensible thing for us to do would be either:

• Continue to raise an error here, because it's an invalid HTTP header value.
• Silently strip any trailing whitespace in HTTP header values.

Although I'd be curious to hear if there's any valid use cases where folks actually need the trailing whitespace for some reason.