Add tests for correct handling of Expect: 100-continue (#138)

Issue #129 prompted me to look into how go-httpbin actually handles
incoming requests that specify an `Expect: 100-continue` header. Here we
add additional unit tests to explicitly codify the, uh, _expected_
behavior for those requests.

Note that the Go stdlib http client and server implementations have
automagical `Expect: 100-continue` handling, which means
these particular tests need to be written at a slightly lower level, by
directly writing to and reading from the underlying TCP connection.

Now, it may well be that the the moving parts (e.g. load balancers)
between a client and the instances of go-httpbin deployed at
https://httpbingo.org intervene and change this behavior, but these
tests give me some confidence that, at least when such requests are made
directly to go-httpbin itself, we are correctly handling them.