api_request method appears to be incompatible with Ruby 1.9

[cwjohnston]

Reported via IRC, it appears that Ruby 1.9 expects only a string argument for the URI when creating Net::HTTP objects. Passing a URI::HTTP object as we currently do causes an exception:

{
  "timestamp": "2016-09-21T10:07:19.753155-0700",
  "level": "info",
  "message": "handler output",
  "handler": {
    "command": "/etc/sensu/mysitehandlers/pagerduty.rb",
    "type": "pipe",
    "severities": [
      "critical"
    ],
    "name": "pagerduty"
  },
  "output": [
    "/usr/lib/ruby/1.9.1/net/http.rb:1860:in `initialize': undefined method `empty?' for #<URI::HTTP:0x0000000251ac78> (NoMethodError)\n",
    "\tfrom /usr/lib/ruby/1.9.1/net/http.rb:2093:in `initialize'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:136:in `new'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:136:in `api_request'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:179:in `stash_exists?'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:191:in `block (2 levels) in filter_silenced'\n",
    "\tfrom /usr/lib/ruby/1.9.1/timmysitet.rb:68:in `timmysitet'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:190:in `block in filter_silenced'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:188:in `each'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:188:in `filter_silenced'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:36:in `filter'\n",
    "\tfrom /var/lib/gems/1.9.1/gems/sensu-plugin-1.4.2/lib/sensu-handler.rb:80:in `block in <class:Handler>'\n",
    "warning: event filtering in sensu-plugin is deprecated, see http://bit.ly/sensu-plugin\n"
  ]
}