Fix for old browsers that don't support ArrayBufferViews in Blob constructor

Unfortunately, I found another problem. Safari 6 and perhaps others supported the Blob constructor but not BlobBuilder. Additionally, ArrayBufferViews in the Blob constructor were broken or not supported. This caused Blob to be undefined in Safari 6, when it really shouldn't be.

This separates out the check for ArrayBufferView support in the Blob constructor and maps them to ArrayBuffers in the Blob constructor when necessary, in addition to my previous PR for BlobBuilder support.