test(connection): increasing test coverage of connection module

[winding-lines]

Work in progress :)

[coveralls]

Coverage increased (+0.23%) to 88.3% when pulling 0717389 on winding-lines:connection-cov into 221f2c9 on hyperium:master.

[coveralls]

Coverage increased (+1.4%) to 89.47% when pulling 8852b4d on winding-lines:connection-cov into 221f2c9 on hyperium:master.

[winding-lines]

@seanmonstar, I am done with mychanges once c1a8e3b gets built. We may have some overlap between tests and bench_header!(), but I think that's ok since they have different goals. Let me know if you have any feedback.

[coveralls]

Coverage increased (+0.29%) to 89.47% when pulling c1a8e3b on winding-lines:connection-cov into 9dc8527 on hyperium:master.