ninja&make/mac: Fix .strings encoding regression from r1707.

Python's decode() function doesn't remove the BOM when decoding 'UTF-16LE' and
'UTF-16BE', so gyp-mac-tool ended up writing two BOMs: The correct BOM, and then
the BOM from the input file.  Fix, add test.

BUG=chromium:280718
R=mark@chromium.org, rsesek@chromium.org

Review URL: https://codereview.chromium.org/23688005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1718 78cadc50-ecff-11dd-a971-7dbc132099af