rtm_send_message fails on newly opened IMs

[elementc]

Replication:

1. Generate a new bot api key.
2. Run the following script:

from slackclient import SlackClient

def search(dict_list, key, value):
    for item in dict_list:
        if item[key] == value:
            return item

SLACK_DEBUG_KEY = "YOUR-BRAND-NEW-SLACK-DEBUG-KEY-HERE"
sc = SlackClient(SLACK_DEBUG_KEY)
sc.rtm_connect()
users = sc.api_call('users.list')['members']
user = search(users, 'name', 'your-slack-username-here')
dm = sc.api_call('im.open', user=user['id'])['channel']['id']
sc.rtm_send_message(dm, 'this should be a message you receive')

Expected:

A message from the bot whose key you used on the first time.

Actual:

This error the first time:

Traceback:
  File "~/pycharmprojects/python-slackclient/slackclient/_client.py", line 40, in rtm_send_message
    return self.server.channels.find(channel).send_message(message)
AttributeError: 'NoneType' object has no attribute 'send_message'

Works as expected the second time.

Notes:

rtm_send_message depends on the cached set of channels. Fix will be in rtm_send_message definition, probably by inserting or creating a channel object if not there. But it's four in the morning here and I'm gonna get some sleep first.

[elementc]

I've posted #74, which fixes both this and #47. Could someone from Slack ( @l12s ?) take a look at this? I know I've posted a lot of PRs this weekend, but #74 is an important one.

[ghost]

Looking now.

[elementc]

Thanks for the merge! Have fun with 1.0.0