Need to sort out the behaviour for status_message with unknown status codes

@oalders I'm unclear where we are on this.

The status_message should never be undef for a code between 100 and 599, and the RFC recommends some fallback values depending on the code range.

Only the numeric code value is to be used for understanding the response. The status code message is intended as a human-readable value, and the values given are standard defaults, but the web client is free to change it to be something else.

A problem is that some downstream modules that use this do not handle undefined values for this, which can cause further problems.

We should be able to have default values, so that newer- or app-specific status codes can be used without having to break anything.

If there is a need to check for known status codes, that should be a separate function.

Start by locating the status_message implementation and any tests covering unknown HTTP status codes. Compare its behavior for codes 100–599 with the issue's fallback requirements, then add coverage showing that the value is defined while preserving numeric-code handling and separate known-code checks.