More lazy loading in HTTPServerRequest

[wilzbach]

Similarly to #1677

Some values are actually a bit problematic thanks to D's type concept (mutable methods!), for example consider the method fullURL method:

@property URL fullURL()
const @safe {
     URL url;
...
	url.username = this.username; //  Error: mutable method vibe.http.server.HTTPServerRequest.username is not callable using a const object
	url.password = this.password; //  Error: mutable method vibe.http.server.HTTPServerRequest.password is not callable using a const object
        return url;
}

I initially had the idea to hack the type system with something like assumeConst:

import std.traits;
private auto assumeConst(T)(T t) @trusted
    if (isFunctionPointer!T || isDelegate!T)
{
    enum attrs = functionAttributes!T | FunctionAttribute.const_;
    return cast(SetFunctionAttributes!(T, functionLinkage!T, attrs)) t;
}

But. of course, without success

[s-ludwig]

If I'm not overlooking something, I think this needs to set _form to .init first, or it would try to access the null field.

[wilzbach]

If I'm not overlooking something, I think this needs to set _form to .init first, or it would try to access the null field.

Excellent catch! Fixed.