HTTPClient.Request.url is a let constant

[weissi]

HTTPClient.Request is a value type (as it should be). Its properties should be mutable (var).
Otherwise a mere

var request = request
request.url = newURL

becomes a tedious

var request = request
request =HTTPClient.Request(url: newURL,
                               method: request.method,
                               headers: request.headers,
                               body: request.body)

[Lukasa]

Relevant prior conversations:

• refactor to make request non-throwing #56 (comment)
• improve request validation #67 (comment)