POST request Content-type headers don't accept extra args

When building an XHR request to MapServer with a POST method some browser will add the charet to the Content-type header parameters. This make MapServer unable to read the map parameter from the post data. 

Example:
```
Ext.Ajax.request({
      url: mapserver,
      method: 'POST',
      params: {
        savequery: 'true',
        map: mapfile,
        mapext: extent.left +" "+ extent.bottom +" "+ extent.right +" "+ extent.top,
        mode: 'nquery',
        ....
      },
      success: function(response) {
        console.log(response)
      }
    });
```

This will make Firefox to send the following content-type:
```
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
```

https://github.com/mapserver/mapserver/blob/master/cgiutil.c#L162
MapServer requires the Content-Type of any POST request to be exactly 
application/x-www-form-urlencoded
Using a strncmp would solve the issue.

In the mean time, the POST request will work if you pass the mapfile in the QUERY_STRING (?map=...mapfile.map).