I am making a request like so in Fiddler2
User-Agent: Fiddler
Host: asdf.example.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 0
Key=asdf:qwer
When I click Execute, Fiddler edits the last line to read:
Key=asdf: qwer
Note the additional space.
Why is this happening and could it cause problems with my request?
Each header field consists of a name followed by a colon (":") and the field value. Field names are case-insensitive. The field value MAY be preceded by any amount of LWS, though a single SP is preferred.
[...]
The field-content does not include any leading or trailing LWS: linear white space occurring before the first non-whitespace character of the field-value or after the last non-whitespace character of the field-value. Such leading or trailing LWS MAY be removed without changing the semantics of the field value. Any LWS that occurs between field-content MAY be replaced with a single SP before interpreting the field value or forwarding the message downstream.
In other words: leading whitespace is to be ignored for the field value, and a space is even preferred. When you do want to send a space, you'll have to quote the string: Some-Header: " foo".
So it's nice of Fiddler to display (and probably send) it like that, though a custom HTTP server that doesn't expect a space there is faulty and should be repaired.
As for your comment regarding the "invalid header name" error the server returns: an HTTP header is defined as such:
message-header = field-name ":" [ field-value ]
field-name = token
field-value = [...]
As you can see, field-name can only exist of token, which does not include = (as that is a separator).
So the header name Key=asdf you use is invalid and the server throws a 400 Bad Request because of malformed syntax. The more specific Invalid header name you claim to get, sounds like you're running your site in IIS. Change the = to - for example, and you'll see it'll work.