[TIMOB-12394] Mobile Web: Ti.Blob does not do Base64 encoding/decoding as specified in the docs

The Titanium documentation [implies](http://docs.appcelerator.com/titanium/2.1/#!/api/Titanium.Utils-method-base64encode) that Mobile Web Blobs store their data in the Base64 form: {quote}On Mobile Web, if obj is a [Titanium.Blob] containing binary data, then obj is returned unmodified because the binary blob is already base64 encoded.{quote} However, the current implementation of Ti.Blob on Mobile Web does not seem to apply Base64 encoding/decoding. Also, Titanium.Network.HTTPClient returns unencoded blobs (with raw binary data). A short code example follows. Expected behavior: 'xhr.responseData' is Base64-encoded. Actual behavior: 'xhr.responseData' is not Base64-encoded.

    var xhr = Titanium.Network.createHTTPClient();
     
    xhr.onload = function()
    {
        var blob = xhr.responseData;  // should be Base64-encoded, but isn't
         
        // Loading image from blob to an image view will fail, because "blob" is not Base64-encoded
        // imageView1.image = blob;
    };
     
    downloadButton.addEventListener('click',function() {
        xhr.open('GET','/images/a-logo.png');
        xhr.file = Ti.Filesystem.applicationDataDirectory + 'a-logo.png';
        xhr.send();
    });