Fix defect in TcpInput where default regexp config delimiter is clobbered

This patch resolves a defect that causes the regexp default
config delimiter to always be clobbered, now we check before we step.

	modified:   plugins/tcp/tcp_input.go


Here is the manual test case:

    [TcpInput]
    address = "127.0.0.1:5565"
    parser_type = "regexp"

    [STDOUT]
    type = "LogOutput"
    message_matcher = "TRUE"
    payload_only = true

and I used netcat to test:

    nc 127.0.0.1 5565
    hello, heka!

I tried to run the unit tests in the code base with `go test` but I got errors about missing libraries ...
Ok, I learned from reviewing travisci output that I needed ` sudo apt-get install protobuf-compiler` in order to run `go test`.