Prevent garbage from coming out of the LTSV parser

[threewordphrase]

Which issue(s) this PR fixes:
I have not opened one yet.

What this PR does / why we need it:
The LTSV parser does not check whether the delimiter is in the pair before it tries to parse it. With delimiter_pattern /\s+/ and label_delimiter =,If I give it the value:

"Sylvanus_007               : ok=45   changed=6"

it parses it like this:

{ "Sylvanus_007":null,":":null,"ok":"45","changed":"6" }

Both Sylvanus_007: null and ":": null were never specified by key=value. This PR prevents this drastically wrong behavior and parses it like this:

{ "ok":"45","changed":"6" }

Docs Changes:
None

Release Note:
None

[threewordphrase]

Not sure what the CI failure is about - I ran that test module per the README, but locally for me bundle exec rake test TEST=test/plugin/test_in_tail.rb passes fine. I cloned the repo fresh just to work on this PR. Ideas?

[repeatedly]

Thanks for the patch!

Not sure what the CI failure is about

This random failure is caused by travis-ci performance/machine. No problem.

[threewordphrase]

Oh wow - I was not expecting this to be merged to master within 24 hours of starting on it. Thank you, fluentd maintainers! When will this version be available on rubygems?