CommandHandler - Check if update meant to be sent to the bot - Case-sensitivity #566

When the bot registered its username as @ThisIsMyBot, it should be case insensitive in any usage.
But now when commands meant to be sent to the bot with different cases of that from getMe(), it gets omitted.

Reference: #515

Steps to reproduce

1. Put multiple bots in a group, here for example @NotMyBot and @ThisIsMyBot (assume all bots have access to all messages)

2. Then use different kinds of command patterns:

• (a) /command
• (b) /command@NotMyBot
• (c) /command@ThisIsMyBot
• (d) /command@thisismybot

Expected behaviour

• (a) Both bots should receive /command
• (b) The command was not meant for my bot, so omitted.
• (c) The command was meant for my bot, so receiving the update.
• (d) The command was meant for my bot (even with different cases), so receiving the update.

Actual behaviour

• (a) Received, correct
• (b) Omiited, correct
• (c) Received, correct
• (d) The command got omitted

Configuration

Operating System:
Ubuntu 16.04 LTS

Version of Python, python-telegram-bot & dependencies:

python-telegram-bot 5.3.0
urllib3 1.20
certifi 2017.01.23
future 0.16.0
Python 3.5.2 (default, Nov 17 2016, 17:05:23)  [GCC 5.4.0 20160609]

Logs