Encode http content as UTF-8 before regex matching

Fix https://github.com/Valarissa/transgress/issues/3 by encoding the response body as UTF-8. 

I googled around and found a [question on SO](http://stackoverflow.com/q/2982677/119768) that suggested  `the net/http library doesn't have any encoding specific options and the stuff that comes in is basically not properly tagged.`. From that I made the assumption that we need to encode the body, and doing that does solve the issue I saw.