hipchat: escape channel name if it is not escaped yet

[barbosa]

As mentioned in #302, if you have a channel name with whitespace in it, the action fails:

irb(main):003:0> URI.parse("https://api.hipchat.com/v2/user/Mobile Engineering/message")
URI::InvalidURIError: bad URI(is not URI?): https://api.hipchat.com/v2/user/Mobile Engineering/message
    from /Users/gustavo/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/uri/common.rb:176:in `split'
    from /Users/gustavo/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/uri/common.rb:211:in `parse'
    from /Users/gustavo/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/uri/common.rb:747:in `parse'
    from (irb):3
    from /Users/gustavo/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'

The current workaround is to set the channel name as an already escaped string, like Mobile%20Engineering.

This PR checks if the channel name is already escaped (so developers using the workaround won't be affected) and escapes it otherwise.

irb(main):004:0> channel = "Mobile Engineering"
=> "Mobile Engineering"
irb(main):005:0> escaped_channel = URI.unescape(channel) == channel ? URI.escape(channel) : channel
=> "Mobile%20Engineering"
irb(main):006:0> channel = "Mobile%20Engineering"
=> "Mobile%20Engineering"
irb(main):007:0> escaped_channel = URI.unescape(channel) == channel ? URI.escape(channel) : channel
=> "Mobile%20Engineering"

fixes #302

[asfalcone]

@barbosa Thanks 👍