Add clj unit tests for org.httpkit.client.Decoder

The Decoder class unexpectedly fails to decode a response starting with

    "HTTP/1.1 200 \r\n"

while the following initial line is accepted even if, strictly speaking,
against the RFC 2616:

    "HTTP/1.1 200\r\n"

(I think both should be accepted by HTTP Kit.)
Fix the client handling of empty Reason-Phrase

Use an `offset` argument with `HttpUtils/findEndOfString` to make it behave
consistently with the rest of the `find*` string methods.
Merge pull request #131 from pyrtsa/bugfix/allow-empty-reason-phrase

Bugfix: Allow empty Reason-Phrase