fix multiple JDBC libraries bug

Author: b1tc4b

docs/filter-jdbc_streaming.asciidoc

@@ -178,7 +178,8 @@ JDBC driver class to load, for example "oracle.jdbc.OracleDriver" or "org.apache
   * Value type is <<path,path>>
   * There is no default value for this setting.
 
-JDBC driver library path to third party driver library.
+JDBC driver library path to third party driver library. In case of multiple libraries being
+required you can pass them separated by a comma.
 
 [id="plugins-{type}s-{plugin}-jdbc_password"]
 ===== `jdbc_password`

lib/logstash/plugin_mixins/jdbc_streaming.rb

@@ -32,7 +32,7 @@ def self.included(base)
   public
   def setup_jdbc_config
     # JDBC driver library path to third party driver library.
-    config :jdbc_driver_library, :validate => :path
+    config :jdbc_driver_library, :validate => :string
 
     # JDBC driver class to load, for example "oracle.jdbc.OracleDriver" or "org.apache.derby.jdbc.ClientDriver"
     config :jdbc_driver_class, :validate => :string, :required => true