Suppress encoding related warnings

[okkez]

This change suppresses warning messages about 800 lines on CI.

lib/fluent/engine.rb:61: warning: setting Encoding.default_internal
lib/fluent/engine.rb:62: warning: setting Encoding.default_external
(about 400 times repeat)
lib/fluent/engine.rb:61: warning: setting Encoding.default_internal
lib/fluent/engine.rb:62: warning: setting Encoding.default_external

[okkez]

We can treat data as binary when read it or open the file.
The data from Socket is always binary.

We are always careful with data from outside of Fluentd after merge this PR.

[tagomoris]

I think just removing assignments of default_internal/external is breaking change for some cases, and it should NOT be done for compatibility.
IMO we have two options:

• assigning encodings only when Encoding.default_* != Encoding::ASCII_8BIT
• removing assignment for default encodings, and add specifying -E to command line arguments of spawn in supervisor.rb, and enable same thing on rake test

Second options is better for me. @repeatedly any thoughts?

[okkez]

In my local, I've tested second option. It works well on Linux and Windows.

[tagomoris]

LGTM!

[tagomoris]

Merged. Thank you!