[ResourceList] Fix selectMode when programmatically setting selectedItems

[AndrewMusgrave]

WHY are these changes introduced?

Resolves #936

WHAT is this pull request doing?

Sets selectMode when selected items exists in state

How to 🎩

Run the example or create your own, the linked issue has more details on the issue 😄

Copy-paste this code in playground/Playground.tsx:

import React from 'react';
import {Avatar, Card, Label, List, ResourceList, Select, TextStyle} from '@shopify/polaris';
interface State {}

export default class ResourceListExample extends React.Component {
  state = {
    selectedItems: [],
  };

  componentDidMount() {
    this.setState({selectedItems: [256]})
  }

  handleSelectionChange = (selectedItems) => {
    this.setState({selectedItems});
  };

  renderItem = (item) => {
    const {id, url, name, location} = item;
    const media = <Avatar customer size="medium" name={name} />;

    return (
      <ResourceList.Item
        id={id}
        url={url}
        media={media}
        accessibilityLabel={`View details for ${name}`}
      >
        <h3>
          <TextStyle variation="strong">{name}</TextStyle>
        </h3>
        <div>{location}</div>
      </ResourceList.Item>
    );
  };

  render() {
    const resourceName = {
      singular: 'customer',
      plural: 'customers',
    };

    const items = [
      {
        id: 341,
        url: 'customers/341',
        name: 'Mae Jemison',
        location: 'Decatur, USA',
      },
      {
        id: 256,
        url: 'customers/256',
        name: 'Ellen Ochoa',
        location: 'Los Angeles, USA',
      },
    ];

    const promotedBulkActions = [
      {
        content: 'Edit customers',
        onAction: () => console.log('Todo: implement bulk edit'),
      },
    ];

    const bulkActions = [
      {
        content: 'Add tags',
        onAction: () => console.log('Todo: implement bulk add tags'),
      },
      {
        content: 'Remove tags',
        onAction: () => console.log('Todo: implement bulk remove tags'),
      },
      {
        content: 'Delete customers',
        onAction: () => console.log('Todo: implement bulk delete'),
      },
    ];

    return (
      <Card>
        <ResourceList
          resourceName={resourceName}
          items={items}
          renderItem={this.renderItem}
          selectedItems={this.state.selectedItems}
          onSelectionChange={this.handleSelectionChange}
          promotedBulkActions={promotedBulkActions}
          bulkActions={bulkActions}
        />
      </Card>
    );
  }
}

[dleroux]

🎩 good. Can you add this bug fix to the change log?