Fix Issue 10543 - std.algorithm.map incorrectly uses source range length for narrow strings  

Strange, because there was an explicit override for strings to forward length.

In any case, this is now fixed.
deactivate unsupported test

The test was checking decode works with input range. The truth is that this relied on the input range actually providing length. map *used* to erroneously provide length for narrow strings, but this not the case anymore.

Issue has already been reported as 9543:
http://d.puremagic.com/issues/show_bug.cgi?id=9543
Merge pull request #1389 from monarchdodra/mapString

Fix Issue 10543 - std.algorithm.map incorrectly uses source range length...