Fix for issue# 8233.

[jmdavis]

std.array.array fails to compile with ranges of immutable elements which have a length property

This wasn't compiling:

import std.array;

void main()
{
    dstring d = "hello world";
    auto e = array(d);
}

but instead gave this error

/home/jmdavis/dmd2/linux/bin/../../src/phobos/std/array.d(55): Error: result[i]
isn't mutable
q.d(6): Error: template instance std.array.array!(immutable(dchar)[]) error
instantiating

[alexrp]

Looks OK to me.

[jmdavis]

Merged.