Fix the register function

Context: When two or more Fields are used either inside the Conditional component or both
in their own Conditional with the same show condition, only the last
Field receive its initial value set in item attribute via Formol
component.

The cause: when a Field is mounted, the register function is executed to
register the component in transientItem variable. But due to the async
nature of setState, the transientItem between Fields registration stays
the same.

I think this commit is an attempt to fix the bug c12d07c
But it seems to appear again #62

src/Formol.jsx

@@ -169,18 +169,27 @@ export default class Formol extends React.PureComponent {
   }
 
   register(name, element, validator, validityErrors) {
-    const { item, transientItem } = this.state.context
+    const { item } = this.state.context
     this.fields.names.push(name)
     this.fields.elements[name] = element
     this.fields.validators[name] = validator
     this.fields.validityErrors[name] = validityErrors
-    // If field is added as a later date we might need to copy its item value
-    // if it's not present in transientItem
-    if (item[name] !== void 0 && item[transientItem] === void 0) {
-      this.setStateContext({
-        transientItem: { ...transientItem, [name]: item[name] },
-      })
-    }
+
+    this.setState(prevState => {
+      const { context: prevContext } = prevState
+      const { transientItem: prevTransientItem } = prevContext
+
+      const newState = item.hasOwnProperty(name)
+        ? {
+            context: {
+              ...prevContext,
+              transientItem: { ...prevTransientItem, [name]: item[name] },
+            },
+          }
+        : { context: { ...prevContext } }
+
+      return newState
+    })
   }
 
   unregister(name) {