Fix: properly pass (symbolized) sequel opts

[kares]

in case of the jdbc input sequel_opts => ... were being passed with string keys (the streaming plugin symbolizes keys).
this is fine e.g. for passing sequel_opts => { "jdbc_properties" => { ... } } as sequel handles the "jdbc_properties" case but officially it's documented as :jdbc_properties.

string keys turned out problematic when trying to disable quoting using: sequel_opts => { quote_identifiers => false } the option wasn't being picked from the Hash, as Sequel only checks for :quote_identifiers key.

also took this opportunity to introduce some sharing between jdbc and jdbc_streaming plugins as I noticed some of the previously fixed issues (e.g. driver loading) could also affect the (lesser used) jdbc_streaming plugin.

[robbavey]

LGTM - test failure is unrelated, and is due to the travis job trying to pull resources for an expired snapshot. (logstash-plugins/.ci#13)

[kares]

Thanks Rob, just realized this isn't enough for me to be able to set sequel_opts => { quote_identifiers => false }
As false value will come back as "false" (String) and thus considered truthy by Sequel, so need to manually handle booleans.

[robbavey]

LGTM again. Thanks for adding an appropriate test