Dynamic mention options do not always load on the first keystroke

When using asynchronously loaded data, the options are not always displayed after the first keystroke. The onSearch function is still invoked, but the options do not get displayed.

Steps to reproduce:

1. Using the story book example for this repo, on the 'dynamic' example. https://rc-mentions.react-component.now.sh/?selectedKind=rc-mentions&selectedStory=dynamic&full=0&addons=1&stories=1&panelRight=0&addonPanel=storybook%2Factions%2Factions-panel
2. Type @a. Data is fetched for the a search, but nothing is displayed
3. Type b, data is fetched for the ab search, and results are displayed
   3a. Erase b, data is fetched for the a search, and results are displayed

After digging around a little bit in the rc-mentions code, it seems after the first keystroke, the measuring state value is still false, which might be leading to this error.

Start with the Storybook dynamic example and reproduce the first-keystroke failure using the listed @a, b, and erase steps. Then inspect the rc-mentions code around the onSearch handling and measuring state; done means asynchronously loaded options appear after the initial search as well as later searches.