Define private _properties as an instance variable.

When _properties are defined only in XMLHttpRequest.prototype, bad
things happen when one uses several instances of XMLHttpRequest in
parallel. (For example this._properties.responseBuffer points to the
same buffer in all instances.)
Call onreadystatechange every time a chunk of data is received.

HTTP Streaming clients (such as SockJS xhr-streaming transport) depend
on onreadystatechange handler being invoked in readyState=3 (LOADING)
whenever a new chunk of data is available.