fix(start-checking): Ensure that state accepted by check()

Change start-checking.js to ensure the minimum requirements on the state
object are met before passing through to check().

lib/start-checking.js

@@ -6,12 +6,16 @@ internals.getOk = require('./get-ok')
 
 function startChecking (state, options) {
   options = parse(options)
-  handleInterval(state, options)
+  state !== undefined && state.method !== undefined && state.url !== undefined && handleInterval(state, options)
 }
 
 function handleInterval (state, options) {
   var timeout
   var ok = internals.getOk(state)
+  var timeoutHandler = function () {
+    var checkAgain = handleInterval.bind(null, state, options)
+    internals.check(state, options).then(checkAgain, checkAgain)
+  }
 
   if (options.checkTimeout) {
     timeout = options.checkTimeout
@@ -31,18 +35,12 @@ function handleInterval (state, options) {
     // we use setTimeout on purpose, we don't want to send requests each
     // x seconds, but rather set a timeout for x seconds after each response
     // but we use `checkTimeout` as variable as the effect is the same
-    state.checkTimeout = setTimeout(function () {
-      var checkAgain = handleInterval.bind(null, state, options)
-      internals.check(state, options).then(checkAgain, checkAgain)
-    }, timeout)
+    state.checkTimeout = setTimeout(timeoutHandler, timeout)
     return
   }
 
   options.interval = 30000
-  state.checkTimeout = setTimeout(function () {
-    var checkAgain = handleInterval.bind(null, state, options)
-    internals.check(state, options).then(checkAgain, checkAgain)
-  }, 0)
+  state.checkTimeout = setTimeout(timeoutHandler, 0)
 }
 
 function parse (options) {