fix #35 HAR Entry - RequestHeadersSize, RequestBodySize and ResponseHead...

The code change sets the HAR entry fields

 * RequestHeadersSize
 * RequestBodySize
 * ResponseHeadersSize

to the values of the data traffic between proxy and web site (i.e., not between browser and proxy) to have an accurate measurement of data traffic volume generated by a web site.
The statistics of the HAR file can be analyzed with http://www.softwareishard.com/har/viewer/.

Please check the code change and merge if appropriate.