Check all libraries for lookup of mime type

[south37]

In some environments, lookup of mime type fails with one library, but succeeds with another library.
For instance, in my Rails app, lookup of 'svg' succeeds only when ::Rack::Mime is used.

[1] pry(main)> ext = "svg"
=> "svg"
[3] pry(main)> defined?(::Mime::Types)
=> nil
[2] pry(main)> ::Mime::Type.lookup_by_extension(ext)
=> nil
[4] pry(main)> ::Rack::Mime.mime_type(".#{ext}")
=> "image/svg+xml"

I think we should check all libraries until lookup succeeds.
This patch will introduce such a checking.

Related to #322 #323

[PikachuEXE]

Can you add a test case to https://github.com/AssetSync/asset_sync/blob/master/spec/unit/multi_mime_spec.rb