Update source_map_utils tests
Update source_map_utils tests

All tests are verified to work minus https://github.com/rails/sprockets/pull/203

All decoding or encoding tests have been verified independently by running against the source-map node module: https://github.com/mozilla/source-map/

```
// Decoding a source map
var sourceMap = require('source-map');
data = '{ \
        "version":3,\
        "file":"index.min.js",\
        "sources":["script.js"],\
        "names":["hello", "console", "log"],\
        "mappings":"AAEAA,QAASA,MAAK,EAAG,CACfC,OAAAC,IAAA,CAAY,eAAZ,CADe"\
      }';
var smc = new sourceMap.SourceMapConsumer(data);
smc.eachMapping(function(m) {
  console.log(m);
});
```

```
// Encoding a source map

var sourceMap = require('source-map');
var map = new sourceMap.SourceMapGenerator({file: "foo.js.min"});
map.addMapping({
  source: "a.js",
  generated: {
    line:   1,
    column: 0
  },
  original: {
    line:   1,
    column: 0
  }
});

map.addMapping({
  source: "b.js",
  generated: {
    line:   2,
    column: 0
  },
  original: {
    line:   20,
    column: 0
  }
});

var source_map_string = map.toString();
console.log(source_map_string);
```

recursecenter/sprockets,eileencodes/sprockets,masarakki/sprockets,prathamesh-sonpatki/sprockets,rails/sprockets