Fix #249

[NaridaL]

Fixes #249

[NaridaL]

Test failure:

 FAIL  test/lazy-observable.js      
  ● lazy observable pending can be observed

    [mobx] Since strict-mode is enabled, changing observed observable values outside actions 
is not allowed. Please wrap the code in an `action` if this change is intended. Tried to modify: ObservableValue@14

      56 |         if (!started) {
      57 |             started = true
    > 58 |             pending.set(true)
         |                     ^
      59 |             fetch((newValue: T) => {
      60 |                 _allowStateChanges(true, () => {
      61 |                     value.set(newValue)

      at invariant (node_modules/mobx/lib/mobx.js:94:15)
      at fail (node_modules/mobx/lib/mobx.js:89:5)
      at checkIfStateModificationsAreAllowed (node_modules/mobx/lib/mobx.js:707:9)
      at ObservableValue.Object.<anonymous>.ObservableValue.prepareNewValue (node_modules/mobx/lib/mobx.js:1017:9)
      at ObservableValue.Object.<anonymous>.ObservableValue.set (node_modules/mobx/lib/mobx.js:1000:25)
      at Object.currentFnc [as current] (src/lazy-observable.ts:58:21)
      at Object.<anonymous> (test/lazy-observable.js:124:8)

[coveralls]

Coverage increased (+0.01%) to 93.5% when pulling d7cf0b5 on fix-249 into 4b59640 on master.

[NaridaL]

@benjamingr Thanks for reviewing.

The continuous-integration/travis-ci/pr job is green on travis but doesn't seem to have updated here.