Fix map handling in .js and .codec.js to match .base.js

The map handling in msgpack.js and msgpack.codec.js only handled
element keys which were of type 0xa0.  This meant any map elements
whose key was not a string of 31 bytes or less would not be imported
correctly.  msgpack.base.js handles element keys correctly, however, so
this commit modifies msgpack.js and msgpack.codec.js to correctly
process map keys.