Fix for https://github.com/yesodweb/wai/issues/131

I think the problem with #131 was that a body beginning with a blank line was messing up the header continuation line detection logic.  I added a bit of code to that logic to check whether the current line is empty or is entirely a carriage return, and if that's the case, then it always reports that there is no continuation.

The test suite still passes, and my tests in the bug report also work with this code.